To clarify the biochemical mechanism responsible for the inhibition by calcitriol (1,25-dihydroxyvitamin D3) of cytotoxicity in peripheral blood lymphocytes (PBL), the human NK sensitive K562 cell line and the human tumor necrosis factor-sensitive murine L929 cell line were used as targets and subsequently compared. The cytotoxicity of PBLs for K562 cells was not changed by preincubation for 4 h with 10 ng/ml phorbol myristate acetate (PMA), but was reduced after an overnight preincubation with 10(-9) or 10(-8) M calcitriol. Using L929 cells, preincubation of PBLs with 10 ng/ml PMA for 4 h increased their cytotoxicity. Overnight incubation with calcitriol significantly reduced PBL cytotoxicity for L929 cells in a dose related manner and suppressed the enhancement of this cytotoxicity by PMA. Pretreatment of PBLs with cycloheximide reduced their cytolysis for L929 cells but did not change their cytotoxicity towards K562 cells. Consequently, the natural cytotoxicity of PBLs for K562 cells does not involve the same mechanism as their cytotoxicity for L929 cells and is therefore subject to different forms of regulation. However, calcitriol reduced PBL cytotoxicity towards both target cells.