A new method for assessing the candidacidal activity of macrophages utilizing neutral red stain gave results comparable to well established vital staining methods, methylene blue and acridine orange, and to the absence of germ tube formation of Candida albicans. Dead yeast cells are uniformly stained red while viable yeast cells are unstained except for a red stained vacuole. This assay was used to demonstrate the difference in candidacidal activity between resident and in vivo stimulated peritoneal macrophages of CBA/J, BALB/c and CFW mice. Stimulation of peritoneal macrophages with Concanavalin A in vivo resulted in a significant increase in candidacidal activity in the macrophages from male and female mice. In addition, Concanavalin A-elicited macrophages from female BALB/c and CFW mice exhibited greater candidacidal activity than macrophages from male BALB/c and CFW mice whereas Concanavalin A-elicited macrophages from CBA/J male and female mice exhibited a similar candidacidal ability. Resident peritoneal macrophages from male mice were significantly less candidacidal than were macrophages from female mice with the exception of the macrophages from male CBA/J mice which were as active as the macrophages from female mice.