The natural cytotoxic effects of peripheral blood lymphocytes (PBL) on Candida stellatoidea and several other Candida species were examined by a colony forming inhibition (CFI) assay. Peripheral blood mononuclear cells (PBMC), were incubated with C. stellattoidea yeast cells. After the incubation period the colony-forming ability of the yeast was significantly reduced. In similar experiments, six different Candida species (C. albicans, C. krusei, C. stellatoidea, C. tropicalis, C. pseudotropicalis, C. guillermondii) were used as target cells. There was no statistically significant difference in the anticandidial activities of PBL against the Candida species used. It was demonstrated that a fraction of lymphocytes, natural killer cells (NK), had the major natural anti-candidial activity by using anti-Leu M1 (CD 15) and anti-Leu 11b (CD 16) monoclonal antibodies (mAbs) plus complement (C'). It was observed that inhibition of colony-forming ability of C. stellatoidea was significantly (78-96%) reduced when anti-leu 11b plus C' were used. In addition, the colony formation inhibition capacity of NK cells was increased by recombinant human interleukin-2 (rhIL-2) while anti-interferon-gamma (IFN-gamma) had no effect. Besides the fact that: NK cells are among those responsible for natural immunity against Candida species, this colony forming inhibition assay performed with C. stellatoidea yeast cells as target and monocyte-depleted PBMC as effector cells, is a simple method to assess NK cell activity.