Possible sex differences and possible age decline of monocyte phagocytosis was studied in healthy blood donors (n = 306). Monocytes from women phagocytosed yeast cells more efficiently than monocytes from men, but the difference was slight. The sex difference was located mainly in the adherence step of phagocytosis. In contrast, no age decline in monocyte phagocytosis could be demonstrated within the age range (18--65 years) represented by healthy blood donors. It is suggested that sex difference and age decline are negligible in the phagocytosis by monocytes from healthy blood donors.