Peritoneal macrophages of F1hybrid mice injected with parental strain spleen cells 8–14 days previously (“early” graft-versus-host (GVH) macrophages) develop an increased ability to destroy in vitro bystanding target cells of syngeneic, allogeneic, and xenogeneic origin. At later stages of GVH reaction, the nonspecific cytotoxicity of macrophages wanes (“late” GVH macrophages) and does not differ much from that of control macrophages. Trypsinization of early GVH macrophages or iodoacetate treatment significantly reduces their cytotoxicity, whereas anti-&thetas; serum and complement have only a moderate effect. Antimouse IgM antibody significantly enhanced the cytotoxic potential of early GVH macrophages, but had no effect either on normal or late GVH macrophages. In contrast, antimouse IgG antibody enhanced, although slightly, only the cytotoxic activity of late GVH macrophages. Two possible explanations of the increased cytotoxicity of early GVH macrophages are offered: (1) cytophilic antibody of IgM type, directed against host antigens, renders macrophages capable of damaging nonspecifically bystanding target cells upon contact with antigen; (2) IgM alloantibody produced by parental cells changes the surface properties of macrophages and contributes to the cytotoxicity of these cells.