Polymorphonuclear leukocytes suspended in Krebs-Ringer phosphate medium ingest paraffin oil containing Oil Red O emulsified with a variety of substances. Spectrophotometric determination of Oil Red O in the cells after uningested particles have been removed by differential centrifugation provides a quantitative measure of phagocytosis. This system has been used to investigate the effects of several drugs and hormones on the initial rate of phagocytosis and to approach the question of how the surface of a particle influences its acceptability as a substrate for phagocytosis. The rate of uptake of paraffin oil emulsified with bovine albumin was constant for 6 min and was proportional to cell concentration when saturating concentrations of paraffin oil emulsion were used. At lower concentrations of substrate, the initial rate of phagocytosis was directly proportional to paraffin oil concentration. The increment in glucose oxidation associated with phagocytosis varied directly with the initial rate of particle uptake. The rate of ingestion of the albumin emulsion was not altered by serum (2-20%, v/v), glucose (5-20 mM), or omission of potassium from the medium. The rate of phagocytosis was decreased 65% if magnesium was omitted, and was essentially zero in the absence of divalent cations. The initial rate of uptake was inhibited by inhibitors of glycolysis, by N-ethylmaleimide (0.05-1 mM), colchicine (0.001-0.1 mM), theophylline (1 and 2 mM), dibutyryl cyclic AMP (1 mM), hydrocortisone (2.1 mM), and ethanol (85 mM). Inhibitors of oxidative phosphorylation and dexamethasone (0.01 mM) were without effect, while insulin (2 mU/ml) slightly stimulated the phagocytic rate. Paraffin oil emulsified with different agents was used to approach the question of how the surface of a particle influences its acceptability as a substrate for phagocytosis. Emulsions prepared with nonionic detergents, methylated proteins, and proteins with a weak net charge at pH 7.4 were poorly ingested. On the other hand emulsions prepared with agents of strong net positive or negative charge were rapidly taken up. The effect of divalent cations on the rate of phagocytosis varied with the nature of the emulsifier, but was not related in any simple, direct fashion to the net surface charge of the particles. However, it has not been conclusively established that charge was the only variable of the emulsion particles employed.