Autoinhibitory activity has been discovered in murine T lymphocyte leukemia models derived from 615 mice in our lab. It was designated 615 mice leukemia associated inhibitor (LAI-615) . To further confirm whether LAI activity could ~ be found in human leukemia, 6 ALL cases and 2 AML cases were examined. The results showed that 5/6 of ALL and 1/2 of AML cases had detectable LAI activity. The different sensitivities of LAI activity were also found between autologous bone marrow cells and human leukemic cell lines, which indicate that autoinhibitory activity might have individual specificity.