The survey of agents that stimulate increases of cAMP and cGMP revealed many similarities but some differences between basophils, mast cells, and neutrophils. With procedures now available for isolation of eosinophils, it will be of great interest to learn how their cyclic nucleotides are regulated. In the granulocytes studied to date, most functions are inhibited by cAMP. In blood basophils and lung mast cells, but apparently not in rat peritoneal mast cells, cGMP can promote release reactions. Neutrophil functions are regulated by cAMP to variable degrees, O2- generation being the most sensitive and phagocytosis perhaps the least. cAMP controls cell surface receptor- and Ca(2+)-dependent events but not those signaled by PKC activation. Very few cytokines have been analyzed for their effects on cyclic nucleotides. LIF and GM-CSF increase cGMP, but more studies are needed to determine whether this effect is relevant to the biological effects of the cytokines. It is conceivable that the clinical efficacy of cytokines could be enhanced by the co-administration of agents tailored to enhance the cytokines' desirable effects on cyclic nucleotides. The study of cAMP, cGMP, and other signaling systems will certainly provide material for exciting research for a considerable time.