Activated peritoneal macrophages from Corynebacterium parvum‐treated mice of most inbred strains, including C57BL/6J (B), are cytotoxic to adherent 1023 sarcoma target cells as well as to larvae of Schistosoma mansoni. Macrophages from A/J (A) strain mice, on the other hand, are defective in this function. Segregation analysis of these two traits was compatible with the hypothesis that effective activation is, in each case, controlled by a single, dominant, autosomal gene. Typing of individual animals of the segregating backcross progeny and of AXB/BXA recombinant inbred strains for the expression of macrophage activation for tumoricidal and schistosomulicidal activity indicates that the genetic control of these two traits is closely linked or identical.