Killing of yeast cells of several species of Candida by murine phagocytic cells was assessed in vitro by a radiolabel release microassay and measurement of colony forming units. The most effective candidacidal phagocytes, i.e. polymorphonuclear and bone marrow cells, were able to kill equally well cells of any species or isolate tested, given sufficient time (4 h) and an appropriate effector: target ratio. However, C. guilliermondii, C. krusei and C. parapsilosis were killed by polymorphonuclear and bone marrow cells much more promptly (1 h) and at a significantly lower effector:target ratio than C. albicans, C. tropicalis and C. viswanathii. Moreover, there were immune effectors such as peritoneal resident macrophages and, mostly, spleen cells which were practically ineffective against C. albicans and C. tropicalis but showed significant activity against C. guilliermondii, C. krusei and C. parapsilosis, even in mice immuno-depressed with cyclophosphamide. Three isolates of C. albicans, differing in the capacity to form germ tubes, also differed in mouse virulence: the germ-tube forming isolate was the most virulent. However, they showed an identical pattern of susceptibility to killing by mouse immunoeffectors, suggesting that virulence is probably not due to the resistance of hyphal cell to phagocytosis.