Recombinant murine gamma-interferon (IFN) and supernatants from concanavalin A (ConA)-stimulated spleen cells were tested for their ability to activate resident peritoneal macrophages (M phi) for fungicidal activity. M phi monolayers pulsed overnight with IFN exhibited significantly enhanced fungicidal activity against Candida albicans (44 +/- 12 versus 0.0%) and Blastomyces dermatitidis (34 +/- 1 versus 3 +/- 3%). The effect of IFN was dose dependent; however, less IFN (10 U/ml) was required to activate M phi to kill phagocytizable C. albicans than to kill nonphagocytizable B. dermatitidis (1,000 U/ml). ConA-stimulated spleen cell supernatants were also able to activate M phi for fungicidal activity against both fungi. The capacity of ConA-stimulated spleen cell supernatants to activate M phi for fungicidal activity was neutralized in the presence of antibody to murine IFN. ConA-treated monolayers acquired the ability to kill C. albicans, but not B. dermatitidis, which was shown to be associated with residual (10%) lymphocytes in the monolayers. Lipopolysaccharide (0.001 to 10 micrograms/ml) failed to consistently activate M phi for fungicidal activity. These data show that IFN can exert an immunoregulatory role on M phi defense against these fungal pathogens.