The Food and Drug Administration's decision last week to ban the antibiotic Baytril in poultry production is among the latest in a series of steps to limit the use of antibiotics in farm animals. The move comes at a time when an increasing number of companies are marketing "antibiotic-free" meat.

The only problem: Many consumers are baffled by what risks antibiotic use in chickens, cows and pigs could pose to human health.

Smithfield Foods Inc., Smithfield, Va., a major pork producer, was expected to announce that it will limit the amount and kind of antibiotics it uses in pigs in compliance with new guidelines imposed by a major customer, Compass Group's North American unit. Food-services giant Compass said it will buy pork and chicken only from suppliers that don't give animals growth-promoting antibiotics that come from classes of drugs also used in human medicine.

A range of other companies, including Whole Foods Market and Chipotle Mexican Grill, a burrito chain majority-owned by McDonald's Corp., advertises that the meat they sell is raised without antibiotics. Murray's Chicken, available in grocery stores with such labels as "no antibiotics administered" or "grown without antibiotics," has increased revenue 20 percent a year for the past seven years, the company says. The number of animals raised under the "Certified Humane Raised and Handled" program, which promises, among other things, that animals are raised with "a healthy diet without added antibiotics," rose to two million in 2004 from 143,000 animals the year before.

But as claims about antibiotic use proliferate, consumers are facing an array of confusing terminology, some vague and some highly technical. Pitches on food labels, Web sites and advertisements range from "antibiotic-free" or "no antibiotic residues," to "without added antibiotics" and "no subtherapeutic antibiotics."

Some of these labels aren't approved by the Department of Agriculture because they are misleading. In fact, no meat sold in the U.S. is allowed to have antibiotic residues, so therefore it is all "antibiotic-free." Because the USDA regulates language only on food labels, many companies get away with using unapproved terms in advertising and on their Web sites.

Such sales pitches and labels may help foster a basic misunderstanding among food shoppers about just why there is concern over the use of antibiotics in farm animals. According to consumers and scientists who specialize in food-safety issues, many people mistakenly think it is because meat and eggs from animals given antibiotics are laced with drugs.

"The problem is not antibiotic residues in the meat. It's in the resistant bacteria that contaminate the meat," says Stuart Levy, the president of Alliance for the Prudent Use of Antibiotics and a microbiologist who wrote a book on antibiotic resistance.

The problem of antibiotic resistance occurs when use of an antibiotic kills off all the susceptible bacteria but leaves behind a few that were able to withstand the drug. These resistant bacteria then can multiply, creating a new race of "super bugs" that drugs can't kill. Consumers and health groups have said that use of antibiotics in farm animals will create more resistant bacteria that live in the animals -- and that could infect someone who later eats the animal. In addition, resistant bacteria in animals can make their way into the environment through ground water, manure and other channels.

Antibiotics are used in livestock production in two distinct ways. One is subtherapeutic antibiotics, which are mixed in with feed and given to farm animals throughout their lives, even when they aren't sick. These antibiotics both prevent disease and promote faster growth for reasons that aren't entirely understood, but may have to do with enhancing the animals' immune systems. The other way the drugs are used is therapeutically, when animals get sick. Farm animals get lots of bacterial infections, for the same reasons school children do: They spend a lot of time together in close quarters where disease spreads easily.

On meat and poultry, labels that say "raised without the use of antibiotics" or "no antibiotics administered" (such as Murray's Chicken) assure buyers that the animals didn't consume either subtherapeutics or therapeutics. Producers that use these labels, which are approved by the USDA, meet the strictest standards for nonuse of antibiotics.

More common are labels that indicate the producer has limited either how or what kind of antibiotics were used. Among claims approved by the USDA: Products labeled as "Certified Humane" indicate that animals weren't given antibiotics subtherapeutically; they may, however, have been treated therapeutically. Labels on beef from specialty-meat producer Niman Ranch, based in Oakland, Calif., say cattle the meat comes from were "never given growth-promoting antibiotics," meaning no subtherapeutic drugs were given, but the animals were treated if they got sick.

Foster Farms, a poultry company based in Livingston, Calif., limits itself to subtherapeutic antibiotics that aren't also used in human medicine.

Not everybody agrees that banning antibiotics is the necessarily right thing to do. Denmark, the world's largest pork exporter, has banned the use of subtherapeutics. Since then, overall antibiotic use in animals has fallen by about half, but therapeutic antibiotics have increased 30 percent to 40 percent as a direct result of the ban, according to the Danish Institute for Food and Veterinary Research. Paul Sundberg, vice president of science and technology for the U.S. National Pork Board, says the Danish experience shows that taking away subtherapeutics leads to sicker animals, which then need to be treated with more therapeutics.

In the case of Baytril, the FDA says the drug still can be used to treat sick cattle and other animals. Because it is typically given to flocks of chicken and turkeys through drinking water, scientists are concerned that it is dispersed too broadly into the environment. Baytril already has been phased out of use by many big chicken producers, including Tyson Foods, and never was used by Perdue Farms, the companies say.

The best way to protect against resistant bacteria and even nonresistant bacteria is to handle and cook meat properly. For information about safe food handling, see www.foodsafety.gov.