The transfer of materials between membranous organelles is believed to be mediated in many cases by vesicles. The regulation of this process is unknown. We shall investigate how the fates of vesicles are controlled by studying pinocytic vesicles in polymorphonuclear leukocytes (PMNs). We find that the chemotactic peptide N-formyl-norleucylleuclyphenylalanine (FNLLP), when taken up into PMNs by pinocytosis, has at least three fates. We hypothesize that these fates are determined by the fusion of the pinocytic vesicles with different intracellular compartments: the plasma membrane, resulting in the release of intact peptide into the medium; and azurophilic granule, resulting in peptide digestion; or a specific granule, resulting in peptide storage. Electron microscopic studies using colloidal gold support this hypothesis. We have developed conditions which allow fusion of pinocytic vesicles with only the plasma membrane. We propose to investigate these and other conditions that modify the fate of vesicles. These studies should reveal factors that regulate the fusion process. We shall also investigate whether vesicle membrane and content follow the same or different intracellular pathways, and we shall examine the topography of pinosome formation and membrane return on locomoting PMNs. Our combined biochemical and morphological approach in an advantageous cell system should provide new and exciting information on basic problems including: vesicle communication, membrane recycling, and cell locomotion as well as the functioning of PMNs on the inflammatory process.