Heterologous antimaerophage serum (AMS) and antilymphoeyte serum (ALS) were prepared by immunizing rabbits with mouse peritoneal cells and lymphocytes, respectively. In vitro tests demonstrate the specific cytotoxic effect of AMS on peritoneal cells and of ALS on lymphocytes. In vivo peripheral leukocyte counts are reduced after a single injection of AMS or ALS. Treatment of allograft recipients with AMS enhances allograft survival to the same degree as does treatment with ALS.