The biochemical basis of peritoneal cell cytotoxicity for Cryptococcus neoformans was studied by measuring the killing of the yeast by peritoneal resident cells and peritoneal exudate cells obtained from normal and proteose-peptone-injected animals, respectively. Both cell populations killed C. neoformans to an equivalent extent after 3 h incubation. Exudate cells showed anti-cryptococcal activity from the first hour of incubation, while no killing was observed with resident cells before 3 h. Both cell populations triggered a respiratory burst in response to opsonized C. neoformans as indicated by the fact that killing of the yeast was inhibited by scavengers of reactive oxygen intermediates (ROI). C. neoformans susceptibility to H2O2 and hydroxyl radicals in cell-free systems is demonstrated by incubating a yeast suspension with different concentrations of H2O2 and Fenton's reagents, respectively. These results suggest that oxygen metabolites play an active role in C. neoformans killing.