Abstract Interferons (IF) have been shown to have a complex regulatory effect on cell-mediated cytotoxicity. In this paper, we described our analysis of the protective effect of IF on human fibroblast target cells. Fibroblasts that have been preincubated with IF are not lysed by natural killer (NK) cells. This IF-induced protection is not observed in antibody-dependent cell-mediated cytotoxicity nor in complement-dependent lysis. Both types of leukocyte IF, viral and immune, and fibroblast IF protect fibroblasts from NK cell-mediated lysis. Electrophoretically purified viral type leukocyte IF also induces protection. Experiments of absorption of NK cells on target cell monolayers and analysis of cytotoxicity at the single-cell level in semisolid medium demonstrate that NK cells bind to both IF-treated and untreated fibroblasts but lyse only the latter. The cytotoxic ability of NK cells is inactivated after interaction with fibroblasts but not with IF-treated fibroblasts. Unlabeled normal fibroblasts, but not IF-treated fibroblasts, compete for the cytotoxicity mediated by NK cells on 51Cr-labeled target fibroblasts. Based on these findings, we propose a possible interpretation of the paradoxic existence of cells that can kill normal cells in the organism and still function in cell-mediated immunity to protect against virus-infected or tumor cells.