Graft-vs.-host (GVH) reactions were induced in adult F1 hybrid mice with the i.p. injection of parental strain spleen cells. Peritoneal exudate and spleen cells of the F1 hybrids taken 8 days after the induction of GVH reaction had a nonspecific in vitro cytotoxic effect which was measured by using 51Cr-labeled target cells of parental genotype [mouse P815X2 mastocytoma cells, mouse sarcoma I cells and mouse L929 fibroblast cells]. The cytotoxic cells in the peritoneal exudates were macrophages which adhered to plastic surfaces and were sensitive to the toxic action of crystalline silica particles. The injection of partially purified syngeneic macrophages into the F1 hybrids undergoing GVH reactions increased the cytotoxic activity of the peritoneal exudate cells obtained from these animals. During GVH reaction, host macrophages are probably activated into a state of nonspecific cytotoxicity.