 Stimulation of phagocytosis is obtained with polymorphonuclear leukocyte (PMN) cells from human, dog, rabbit and cow as well as with macrophages from the lung and peritoneal cavity of mice, and guinea pig and mouse bone marrow cells.This effect is inhibited by peptide analogue Thr-Lys-Pro-Pro-Arg.Basal activity is not inhibited, so basal phagocytosis may follow a different pathway from that which follows stimulation.