A new method for the measurement of phagocytosis of Candida albicans by human polymorphonuclear leucocytes (PMN) is described using a fluorescence activated cell sorter. We have used acridine orange to discriminate between PMN which have internalised yeast particles and those which have not. This method allows accurate measurement of particle phagocytosis as an event distinct from particle adherence. It also permits detailed examination of the kinetics of phagocytosis, the study of which is likely to be of value in the investigation of diseases where abnormalities of PMN function are suspected.