We have previously described the low virulence of a Candida albicans morphological mutant: 92'. We have now used this strain to examine the role of phagocytes in its pathogenesis. Our results show that C. albicans 92' cannot evade innate host macrophage defence mechanisms as efficiently as the parental strain. In addition to the high susceptibility to phagocytosis by peritoneal macrophages, the NO produced by macrophages is a very important element in the low virulence of this agerminative mutant, a thesis supported by in vivo and in vitro experiments. Whereas the parental strain was able to inhibit macrophage NO production, the mutant was quite inefficient at reducing NO production by macrophages. In addition, the mutant showed high sensitivity to a NO generator. Treatment of mice with aminoguanidine (a preferred inducible NO synthase inhibitor) caused 90% mortality in 92' systemic infection, thus supporting a role for NO in the low virulence of this strain. Our data show that both the low inhibitory effect of 92' on macrophage NO production and the higher sensitivity to NO underlie the low virulence of this strain.