The interactions with tumor target cells of resident and BCG-activated murine peritoneal macrophages (M phi) as well as of BCG-activated M phi additionally stimulated by a lymphokine-like factor were investigated in order to get some insight into the cytolytic process mediated by activated M phi. The lymphokine-like factor enhancing the cytotoxicity of BCG-activated M phi (MCF) was isolated and partially purified from cell-free fluid of rat Zajdela ascites hepatoma. M phi cytotoxicity was determined by a modified 51Cr release assay. Scanning and transmission electron microscopic findings suggested a two-step mechanism of target cell lysis: a first step of specific attachment of processes of M phi on the target cell surface and a second step with transport of lysosome-like vesicles to the target cells obviously with liberation of these vesicles in the immediate vicinity of target cells resulting in a local accumulation of cytolytic substances. This interpretation was supported by findings after treatment of interacting effector and target cells with amphotericin B and bestatin which substances were modifying M phi cytotoxicity. MCF caused only an augmentation of M phi cytotoxicity without qualitative differences to the cytolytic action of merely BCG-activated M phi.