When lymphocytes from 1 person were reciprocally tested in cell-mediated lympholysis (CML, effectors and stimulators) against a panel of HLA-A and B [bone marrow derived] fully incompatible lymphocytes, this panel was divided into 2 groups. An identical discrimination was obtained both when panel lymphocytes were used as stimulators/targets and when they were used as effectors. Significant lympholysis was obtained in all combinations. This pattern of reaction was found with 3 different, unrelated individuals, each tested against a selected panel. These 3 individuals exhibit different HLA-A and B phenotypes. These findings were reproducible and indicate the existence of mechanisms modifying the otherwise marked influence of the HLA-A and B antigens in indirect CML.