Preincubation of a cloned cell line with natural killer (NK) cell activity, as well as splenic mononuclear cells with either delta 9-tetrahydrocannabinol (THC) or 11-hydroxy-delta 9-tetrahydrocannabinol (11-OH-THC) suppressed NK cytolytic activity against YAC-1 target cells in a dose-dependent manner. THC was more inhibitory for cloned cells than 11-OH-THC and suppressed the lytic activity of these cells without reducing cell viability in the concentration range of 5 micrograms/ml (16 microM) to 10 micrograms/ml (32 microM). THC also inhibited proliferation of cloned NK cells, but this inhibitory effect was reversible in that extensive washing of cells following cannabinoid pretreatment eliminated the suppressive effect. Single-cell analysis revealed that THC did not inhibit the binding of cloned NK cells to target cells and further showed that NK cells freshly isolated from mouse spleen were restricted in killing capacity following binding to target cells. Therefore, THC and 11-OH-THC appear to directly inhibit NK cell cytolytic activity in a postbinding stage.