Doctors report they've found a way to alert the body's immune system to hunt down cancer cells after bone marrow transplants for leukemia in an effort to wipe out lingering traces of the disease.

The new technique, still highly experimental, uses the common transplant drug cyclosporine to confuse disease-killing blood cells so they attack the patients' own body, believing it is foreign."Nobody really understands the phenomenon yet, except that it appears to have an anti-leukemia effect," Dr. Robert B. Geller of the University of Chicago said Tuesday.

Bone marrow transplants were once limited to young patients with blood cancers. But now they are being used in a variety of cancers in steadily older patients.

When people receive marrow from relatives, the transplanted tissue often recognizes its new owner as foreign and attacks his body, causing a potentially life-threatening reaction called graft-vs.-host disease.

For reasons that no one yet understands, cyclosporine makes the blood cells go after particular types of tissue, including skin cells and cancer cells. The result is a mild form of graft-vs.-host disease that usually clears up by itself without serious side effects.