Natural cytotoxicity (mediated by the B73.1+ subset) of human peripheral blood lymphocytes against the K-562 erythroleukaemia cell line is dramatically inhibited in a dose-dependent manner, by the small molecular weight protease inhibitors, tosyl-L-lysyl chloromethyl ketone (TLCK) and tosylamide phenyl-ethyl-chloromethyl-ketone (TPCK), incorporated into the cytotoxicity assay or after brief effector cell (but not target cell) pre-treatment. The alkylating ketones primarily affect post-binding events in the lytic process by interference with cellular functions dependent upon protein synthesis. Although non-toxic under the conditions used, recovery of cytolytic function requires at least 72 h, implicating involvement of protein(s) with a minimum turnover time of 3 days. Protection of effector cell function from TLCK by prior treatment with the lectin Lens culinaris (lentil) agglutinin, which binds human peripheral blood lymphocytes to a three-fold greater extent than concanavalin A, indicated that the initial action of the agent is with cell surface rather than intracytoplasmic components. The data suggest that the alkylating ketones inhibit natural killer function by slowly reversible functional inactivation of cell-surface protease(s), which although not cytotoxic per se, may control the secretion of soluble lytic factors.