Several studies have demonstrated an augmentation of natural killer (NK) cell-mediated cytotoxicity by various adjuvants including BCG. Inhibitory effects of BCG have also been reported, particularly for relatively high doses. Because the cell wall of Mycobacterium bovis BCG contains a high proportion of lipids, the possibility was considered that these lipids may modulate NK activity. A total lipid fraction was extracted from Mycobacterium bovis BCG and used for the lipid modulation of NK effector and target cells. Treatment of effector or target cells resulted in decreased membrane fluidity and decreased NK cell-mediated cytotoxicity in both cases. Pretreatment of target cells did not affect the binding between target and effector cells, as shown in the single cell assay, whereas pretreatment of effectors resulted in inhibition of conjugation. It was further demonstrated that treatment of target cells which were first programmed for lysis protected these cells from subsequent lysis during the killer cell independent lysis stage. The results of this study suggest that adverse effects of BCG treatment on immune functions may be mediated by BCG derived lipids.