The effect of human recombinant interleukin-1 alpha (IL-1 alpha) on a systemic candidal infection in mice under various conditions of immunosuppression was investigated. In normal mice and in mice pretreated with cyclophosphamide, hydrocortisone acetate, or sublethal total body irradiation, the outgrowth of Candida albicans in the kidney was significantly reduced by the administration of a single intraperitoneal dose of 80 ng of IL-1 (P less than 0.05). In mice treated with either cyclophosphamide or irradiation, IL-1 also significantly reduced the outgrowth of C. albicans in the spleen. The protective effect of IL-1 was present when given 24 h before injection of C. albicans but also when IL-1 was given simultaneously with or 6 h after injection of C. albicans in cyclophosphamide-treated mice. The effect of IL-1 was independent of the presence or recruitment of granulocytes, since IL-1 inhibited the outgrowth of C. albicans in the kidneys and spleens of mice that were rendered severely granulocytopenic (less than 50 granulocytes per mm3) throughout the duration of the infection by either repeated injections of cyclophosphamide or sublethal total body irradiation. These results indicate that the enhancement of host resistance by IL-1 is not due solely to increased granulocytopoiesis or chemotaxis of granulocytes but strongly suggest that other mechanisms play a role in the protective effect of IL-1 against systemic infections.