A phosphatidylcholine (PC) fraction isolated from commercial bovine lecithin has been shown to have a stimulatory effect, in terms of carbon clearance, on the reticuloendothelial system (RES). Intraperitoneal injections of 4 mg of PC one day, two days and 3 days prior to carbon clearance assaying for RES activity indicate a 45, 44 and 39% stimulation, respectively, when compared to saline injected controls. This degree of stimulation is determined 13 minutes after intravenous carbon injection. The degrees of RES stimulation observed 13 minutes post carbon injection for the 1, 2 and 3 day post PC injection assays is 75, 63 and 51%, respectively. Carbon clearance assays were also done 1 and 6 hours post PC injection. The results at these two time periods after treatment indicate no significant stimulation in carbon clearance. Overall the results indicate a stimulatory effect of PC on reticuloendothelial activity, with an initial lag period before its action is seen.