Protection against Candida infection involves both innate and acquired immune responses, and cytokines produced by monocytes during the innate response may modify the acquired immune response by T cells. We hypothesized that Candida species which differ in pathogenicity can differentially induce production of immunoregulatory cytokines by human monocytes, which in turn modify T cells for immune responses to Candida. To test this hypothesis, we examined the effects of Candida albicans and Candida krusei on immunoregulatory cytokine production by human monocytes and gamma interferon (IFN-) production by peripheral blood mononuclear cells (PBMC). Purified monocytes were incubated with live or heat-killed strains of C. albicans and C. krusei at the optimal Candida/monocyte ratio of 0.5. Cytokines in the supernatants were measured by enzyme-linked immunosorbent assay. Our data demonstrated that live C. albicans and C. krusei significantly induced interleukin-10 (IL-10), monocyte chemotactic factor 1, IL-1, and tumor necrosis factor alpha production by monocytes relative to unstimulated monocytes. In contrast, unlike C. krusei, pathogenic live strains of C. albicans induced no or only a minimal level of IL-12. The expression of IL-12 p40 mRNA levels by reverse transcription-PCR corroborated the IL-12 protein (p70) findings. In human PBMC, human blood monocytes were the major source of both IL-10 and IL-12 production in response to C. albicans and C. krusei. Upon activation of T cells in the presence of Candida-modified monocytes and antigen-presenting cells, IL-12 production by PBMC treated with Candida organisms correlated strongly with the level of IFN- production by T cells. These results indicate that the virulence of C. albicans may be related to its ability to induce the monocytic type II cytokine IL-10, with a selective inhibition of IL-12 production, which may be responsible for the observed lack of T-cell IFN- and may restrain an effective type I immune response to Candida.