Serum from DBA/2J mice bearing a syngeneic T1699 mammary adenocarcinoma contains not only a specific antitumor antibody but also another factor(s) which arm normal monocytes in vitro and transform them into specific killer cells. Arming of normal monocytes by the factor(s) was inhibited by cytochalasin B and dactinomycin but not by mitomycin C. The ADCC activity of monocytes was at least partially resistant to the action of dactinomycin but cytotoxicity of armed effectors was totally blocked by the same drug. Effective cytolysis by armed monocytes required at least 5–6 h of continuous contact between the target and effector cells; and cytotoxicity could be reversed by cytochalasin B during this perio