A new method is described for measuring phagocytosis in vitro. Leucocytes obtained by peritoneal irrigation of the rabbit were allowed to phagocytose 51Cr-labelled yeast cells. The subsequent 51Cr-activity of the leucocytes was taken as a measure of phagocytic activity. The method was tried in the presence of some factors known to be capable of affecting phagocytic activity. The results obtained agreed well with those reported in previous investigations. The method is suitable for studies of the course of events of phagocytosis.