Single mom Leann Mischel wanted her son to have a sibling -- a full-blooded sibling. But it wasn't looking good. The boy's father was out of the picture, so to speak. In fact, Ms. Mischel was just one of many women waiting on him. The Pennsylvania college professor reluctantly settled for her second choice.

And then Carla Schouten from San Jose had the gift of a lifetime for Ms. Mischel, 41 -- an extra vial of the father's sperm chilling in her doctor's refrigerator. Ms. Schouten's son was fathered by the same donor. The two women met on the Internet and bonded.

Some women have their book clubs, and others belong to professional groups. Some connect in therapy and others through sororities. But here is a relatively new connection: a group of 11 sharp, educated and independent women brought together by one man's sperm.

Not one of them has met the donor -- his identity is kept secret by Fairfax Cryobank in Virginia. He has fathered all of their children -- 11 so far, and Ms. Mischel's second child on the way.

"It's an emotional connection. We have a common base," explained Ms. Schouten, who adds that the women have less interest in knowing the donor than they do one another. "Most of us are single. We all desired children, and we were all attracted to the same donor."

There is much happening in world of sperm banks these days. On the Web site for Fairfax Cryobank -- one of the largest in the country -- a sperm shopper can browse the catalog for Mr. Right Donor. Free of charge, she can find a staff assessment of the man, as well as all sorts of preferred paternal criteria: eye and hair color, nationality, blood type, height and profession. (For Ph.D, MD and attorney sperm, there is premium charge of $425 a sample.) Donors make a six-month commitment and are paid $900 to $1,500 a month.

There's also much more detail available for various fees: baby pictures, an audiotape and extensive medical history. Most everybody buys everything; the donor's name, birth date, address and other specific identifying information are not available.

But recently a clever 15-year-old boy figured out how to piece together his biological father's identity. The world of anonymous sperm donors gasped -- and administrators took a second look at policies.

As best can be determined by the birth dates of the 11 babies, Mr. 401 got into the baby business at Fairfax Cryobank about six years ago -- and what a popular fellow he turned out to be. Here's what we know about him: There is a good chance he lives in the Washington metropolitan area; he is 6-foot-4, of German heritage, has a master's degree, is athletic and is very close to his mother. "A ray of light," he called her in his personal essay.

It was one of those sweet details about a man that most women couldn't walk away from. "Such a nice guy," said Ms. Schouten, 43.

"He was tall and so am I," explained Carolyn George, 34, of Oklahoma, whose youngest child, Connor, 2, is the biological son of Mr. 401. "He seemed likable."

Around the same time that Mr. 401 was getting into the donating business, a woman who had no connection to Mr. 401 developed a Web site, DonorSiblingRegistry.com, and set out to see if she could locate the sperm donor for her son, 9 at the time.

Wendy Kramer was not successful in her search, but her Web site is booming. Through word of mouth and publicity, it has more than 6,000 members. In addition to the Mr. 401 moms who met on the site, an additional 22 women connected to another single donor also found one another.

The Mr. 401 mothers want to make it very clear that in signing up on the site, they were not looking for a husband or a relationship -- and they are not romanticizing about the donor. They acknowledged that their children might one day be curious, but for now, they are satisfied with not knowing him and having one another.

"I think it would be a little bit weird to meet him now," George said.

"Some doors are better left closed," said Louisa Weix, 43, who has twin girls by Mr. 401. "All of us get a little queasy about the idea of invading someone's privacy."