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N (2)- ( (1- (N (2)-L-Threonyl)-L-lysyl)-L-prolyl)-L-arginine. A tetrapeptide produced in the spleen by enzymatic cleavage of a leukophilic gamma-globulin. It stimulates the phagocytic activity of blood polymorphonuclear leukocytes and neutrophils in particular. The peptide is located in the Fd fragment of the gamma-globulin molecule.