Considerable evidence has accumulated for a role of a nonsteroidal mevalonate product in the regulation of DNA replication and cell division. We report here a similar requirement for mevalonate in a nonreplicative function, that of natural killer (NK) cell cytotoxicity. Treatment of NK cells with 10 microM compactin for 48 hr results in a significant inhibition of cytotoxicity which can be completely reversed by treatment with 1 mM mevalonate, but not cholesterol, dolichol, or isopentenyl adenine. Protein and RNA synthesis appear to be involved in this reversal. Treatment with compactin and reversal with mevalonate do not affect the phenotypic distribution of the effector cell population, and the cell type involved in the inhibition and reversal of cytotoxicity is a CD16 (Leu 11)-, Leu 19-positive, large granular lymphocyte. The conjugation of the target and effector cell early in the lytic pathway is inhibited by compactin treatment of the effector cell population, and this inhibition is reversed by mevalonate.