Synaptophysin is a major glycoprotein of Mr approximately 38,000 (in deglycosylated form: Mr approximately 34,000) characteristic of a certain class of small (30-80 nm diameter) neurosecretory vesicles, including presynaptic vesicles, but also vesicles of various neuroendocrine cells of both neuronal and epithelial phenotype. Using synaptophysin-specific antibodies we have isolated cDNA clones from rat nervous tissue libraries, which identify an approximately 2.5-kb mRNA in rat and human cells, including neuroendocrine tumours, that contains a reading frame for a polypeptide of 307 amino acids with a total mol. wt of 33 312. The deduced amino acid sequence, which was partly confirmed by comparison with sequences of two tryptic peptides obtained from purified synaptophysin, revealed four hydrophobic regions of 24 amino acids each, which are characterized, according to conformation prediction analyses, by marked alpha-helicity. The sequence shows a single potential N-glycosylation site, which is assigned to the vesicle interior, and a carboxy-terminal tail of 89 amino acids which contains glycine-rich tetrapeptide repeats, the epitope of monoclonal antibody SY38, and a number of collagenase-sensitive sites accessible on the surface of the intact vesicles. These features suggest that the polypeptide spans the vesicle membrane four times, with both N and C termini located on the outer, i.e. cytoplasmic, surface of the vesicles.