Narrator: This is Science Today. Organ transplants are a risky business. Dr. Flavio Vincenti of the University of California, San Francisco says that until recently, the chances of surviving a transplant were poor, because without the right drugs, new organs are rejected by the body's immune system.

Vincenti: Up to 1983, we were almost in the dark ages of drug immuno-therapy. We had two main drugs. We had very high rejection rates. Survivals of, for example, kidney transplants were at best 50 percent at one year.

Narrator: Today, the picture is much brighter, but researchers are still looking for better drugs. There was hope a few years ago with the invention of genetically engineered mouse antibodies. Unfortunately, our immune systems recognize a mouse antibody as foreign and reject the drug itself.

Vincenti: And so these mouse antibodies, while helpful to some degree, they cannot be used repeatedly.

Narrator: But now Vincenti and other doctors are testing a new transplant drug that's part mouse and part human. The human part disguises the mouse part, allowing it to work effectively. For Science Today, I'm Steve Tokar.