Cell-mediated immune responses appear to be critical in the outcome of cryptococcosis. Interleukin-12 (IL-12) was studied for its potential use as a therapeutic agent because of its stimulation of natural killer cells and gamma interferon production by stimulated T cells and natural killer cells. Gamma interferon-activated macrophages are important in host resistance against cryptococcosis. In two separate studies, male BALB/c mice were infected intravenously with Cryptococcus neoformans. In the first study, mice received either no treatment, 5.0 mg of fluconazole alone per kg of body weight per day (by gavage twice daily), or IL-12 subcutaneously at 0.01, 0.1, or 1.0 microgram/day once daily (low-dose study) alone or in combination with 5.0 mg of fluconazole per kg/day. In a second study (high dose), the dosages of IL-12 used were 1.0, 2.5, or 5.0 micrograms/day. Therapy was given for 10 consecutive days, and the number of CFU of C. neoformans remaining in various organs was quantitated 1 or 2 days after administration of the last dose. In the low-dose study, IL-12 at 0.1 or 1.0 microgram reduced the level of brain infection by approximately 10-fold (P < 0.05) and IL-12 at 1.0 or 0.1 microgram/day enhanced the efficacy of fluconazole. In liver, both the efficacy of IL-12 alone (0.01 or 0.1 microgram; P < 0.05) and enhancement of the efficacy of fluconazole (P < 0.05) were seen. No efficacy of IL-12 was seen in spleens or lungs, although spleen weights increased fourfold in mice given 1.0 microgram of IL-12 per day. In the high-dose study, all IL-12 doses alone again reduced the levels of brain infection (5- to 8-fold; P < 0.05) when the two were given in combination. No overt toxicities were observed at any dose, and overall, 1.0 microgram of IL-12 per day was found to be the optimal dosage for reducing infection in the brain. To our knowledge, this is the first demonstration of the efficacy of cytokine therapy in systemic and particularly brain infections with C. neoformans. The stimulation of cell-mediated immunity represents a new approach to therapy and can enhance suboptimal antimicrobial chemotherapy. IL-12 should be considered for further study and for clinical trials. These studies suggest that other opportunistic central nervous system pathogens should also be investigated.