The influence of Tuftsin, the synthetic phagocytosis-stimulating tetrapeptide (L-threonyl-L-lysyl-L-prolyl-L-arginine), on the nitrous blue tetrazolium (NBT) reduction by human polymorphonuclear leukocytes was investigated. It was found that this substance increases the NBT reduction by approximately as much as endotoxin. Other tetrapeptides do not share this property. When Tuftsin analogs are added to the cell suspension and incubated, they prevent the action of both Tuftsin and endotoxin but not of methylene blue. When washed of the analogs, the cells regain the property to be activated by both Tuftsin and endotoxin. It appears that methylene blue on one hand and Tuftsin and endotoxin on the other hand have different sites for their actions. We suggest that whereas methylene blue diffuses into the cell and acts directly upon the hexosemonophosphate shunt activation, Tuftsin and endotoxin appear to act on the cell membrane binding to specific receptors. By treating the cells with Tuftsin analogs, we probably block these receptors.