Cytotoxic lymphocytes have the capacity to kill microbes directly; however, the mechanisms involved are poorly understood. Using Cryptococcus neoformans, which causes a potentially fatal fungal infection in HIV-infected patients, our previous studies showed that granulysin is necessary, while perforin is dispensable, for CD8 T lymphocyte fungal killing. By contrast, the mechanisms by which NK cells exert their antimicrobial activity are not clear, and in particular, the contribution of granulysin and perforin to NK-mediated antifungal activity is unknown. Primary human NK cells and a human NK cell line YT were found to constitutively express granulysin and perforin, and possessed anticryptococcal activity, in contrast to CD8 T lymphocytes, which required stimulation. When granulysin protein and mRNA were blocked by granulysin small interfering RNA, the NK cell-mediated antifungal effect was not affected in contrast to the abrogated activity observed in CD8 T lymphocytes. However, when perforin was inhibited by concanamycin A, and silenced using hairpin small interfering RNA, the anticryptococcal activities of NK cells were abrogated. Furthermore, when granulysin and perforin were both inhibited, the anticryptococcal activities of the NK cells were not reduced further than by silencing perforin alone. These results indicate that the antifungal activity is constitutively expressed in NK cells in contrast to CD8 T lymphocytes, in which it requires prior activation, and perforin, but not granulysin, plays the dominant role in NK cell anticryptococcal activity, in contrast to CD8 T lymphocytes, in which granulysin, but not perforin, plays the dominant role in anticryptococcal activity.