The potential of the resident murine Kupffer cell to be cytotoxic in extra- and intracellular killing systems in vitro was investigated. Kupffer cells exerted no spontaneous cytotoxicity but were readily susceptible to activation with lymphokines. Such activated Kupffer cells very efficiently killed extracellular P815 cells and intracellular Leishmania spp. parasites. Kupffer cells could be induced to proliferate in vitro under the influence of colony-stimulating factor (D.-M. Chen, H.-S. Lin, P. Stahl, and R. Stanley, Exp. Cell Res. 121:103-109, 1979). Kupffer cell-derived macrophages cultured in vitro were identical to their liver-derived progenitors in terms of macrophage-specific surface antigens and with respect to extra- and intracellular cytotoxicity. These results demonstrated that Kupffer cells have a strong self-renewing potential and that essential Kupffer cell properties like antigenic determinants and cytotoxic potential remained stable throughout the replicative process. The possibility of Kupffer cell self-renewal in the intact organism is also discussed.