Phagocytic and migratory functions of peritoneal macrophages from Friend virus (FV) leukemic mice are significantly depressed as compared with normal controls. Leukemic macrophages exposed in vivo and in vitro to statolon, an extract of the mold Penicillium stoloniferum, shown previously to suppress FV erythroleukemia, regain normal function and release reduced amounts of FV. Statolon's in vivo restoration of leukemic macrophage function is paralleled by restoration of humoral immune competence. Statolon induces interferon in vitro but its effects on leukemic macrophages are probably direct, since restoration of macrophage function occurs at dosage levels far below those that induce interferon. These studies suggest that macrophages play an integral role in both the pathogenesis and the statolon-induced suppression of FV disease.