Others researchers have found that in some cases, before the twin is absorbed, some of its cells enter the body of the other fetus and remain there for life. The cells can include bone marrow stem cells, the progenitors of blood cells.

Another route to chimerism is through the cells that routinely pass from a mother to fetus and remain there for life.

Dr. Ann Reed, chairwoman of rheumatology research at the Mayo Clinic, who uses sensitive DNA tests to look for chimerism, finds that about 50 to 70 percent of healthy people are chimeras. The more scientists look for chimerism, the more they find it. It seemed not to exist in the past, she said, because no one was explicitly looking for small amounts of foreign cells in people's bodies.

"Some believe that if you look hard enough you can find chimerism in anybody," said Dr. Reed, who also has not been involved in the Hamilton case. It is so common that she thinks there must be a biological reason for it. It also may cause problems, she and others say.

Chimerism may be why bone marrow from a seemingly perfectly matched donor relentlessly attacks a patient who receives it in a transplant -- the attackers may be a small percentage of cells in the marrow that come from someone else. It also may help explain autoimmune diseases, when the body's own immune cells attack. The attacking cells may be the foreign ones that arise from someone else.

The Hamilton case involves a test developed by Dr. Margaret Nelson and her colleagues at the Royal Prince Alfred Hospital in Australia. It was based on a simple idea: if an athlete got a transfusion, he would have to make sure the blood was the right match using the blood antigens A, B and O. But blood cells have other surface proteins, so-called minor antigens, that do not matter in blood typing for transfusions but can be used to distinguish one person's blood from another's. The investigators said they could use a sensitive test, flow cytometry, to search for small amounts of blood with minor antigens different from those in the athlete's own blood.

It was an important advance, anti-doping agency officials said. They knew that athletes, including cyclists, had used blood transfusions in the past to boost their performance but had no test to prove it.