Incubation of human polymorphonuclear leukocytes (PMNL) or thioglycollate-stimulated mouse peritoneal macrophages with the phagocytosis-stimulating peptide, tuftsin (2.5 X 10(-7) M, at 37 degrees C), caused an increase of 80-90% in intracellular cGMP levels, accompanied by a decrease of 20-25% in intracellular cAMP levels. Significant changes in cyclic nucleotide levels were detectable after 4 min of incubation, were maximal at 10-20 min and persisted for at least 60 min. The concentration dependences of the stimulatory effect of tuftsin on modulation on intracellular levels of cyclic nucleotides and on phagocytosis are similar, suggesting a cause and effect relationship between the two phenomena. This notion is further supported by the finding that 8-Br-cGMP and 8-Br-cAMP elicit stimulatory and inhibitory effects on macrophage phagocytosis, respectively. Measurement of 45Ca2+ influx into PMNL and macrophages in the presence and absence of tuftsin did not reveal any change in 45Ca2+ uptake from the media. However, tuftsin did enhance release of 45Ca2+ from cells preloaded with the isotope. Results suggest that modulation of both the amount of cell-associated 45Ca2+ and the intracellular levels of cyclic nucleotides are key steps in the mechanism by which tuftsin augments phagocytosis.