The possible role of platelet-activating factor (PAF) in natural killer (NK) cell cytotoxicity was investigated by examining the effect of the PAF antagonist BN 52021 in NK cytotoxicity towards 51Cr-labelled K 562 target cells. When BN 52021 (30-120 microM) was added during the assay, a dose-dependent inhibition of NK activity was observed. The inhibition of cytotoxicity by BN 52021 was not due to an alteration of the binding of lymphocytes to K 562 cells. When lymphocytes were preincubated with BN 52021 (60 microM) for 60 min before the target cells were added, the inhibitory effect of the drug was similar to that observed when it was added at the start of the reaction. Inhibition was more pronounced when the target cells were pretreated for 60 min before the start of the assay. BN 52021 (60 microM) also inhibited gamma interferon induced NK activity. These studies provide indirect evidence that NK cells can generate PAF and that this mediator is involved in cytotoxic processes.