The functional importance of various cell membrane bound molecules was studied and compared in the NK cytotoxicity and CTL activity. LFA-1 and CD2 participate in both killing functions, while CD3 and CD8/CD4 as well as MHC class I molecules are involved only in CTL activity. Nevertheless CD2- and beta 2-microglobulin are representatives of the NK function. It was demonstrated that CD2-, LFA-1 and beta 2-microglobulin molecules have an additive and complementary function in the killing mechanism. The upregulation of alpha- and gamma-interferons on NK function seems not to be a consequence of the enhanced expression of these molecules on the cell surface induced by IF at the same time.