Tuftsin, T-K-P-R, is a phagocytosis-stimulating peptide described as a natural immunostimulant. Four analogues of this peptide were synthesized. These compounds were assayed for their ability to compete with [3H]tuftsin for its specific receptor from thioglycollate-elicited mouse peritoneal macrophages. They were also tested for their ability to change level in intracellular cGMP and to stimulate phagocytosis through the nitroblue tetrazolium reduction measurement. Surprisingly, all the analogues were poor competitors of [3H]tuftsin binding but possess potent tuftsin-like activities.