Syncollin is a 16-kDa protein that is associated with the luminal surface of the zymogen granule membrane in the pancreatic acinar cell. Detergent-solubilized, purified syncollin migrates on sucrose density gradients as a large (120-kDa) protein, suggesting that it exists naturally as a homo-oligomer. In this study, we investigated the structure of the syncollin oligomer. Chemical cross-linking of syncollin produced a ladder of bands, the sizes of which are consistent with discrete species from monomers up to hexamers. Electron microscopy of negatively stained syncollin revealed doughnut-shaped structures of outer diameter 10 nm and inner diameter 3 nm. Atomic force microscopy (AFM) of syncollin on mica supports at pH 7.6 showed particles of molecular volume 155 nm(3). Smaller particles were observed either at alkaline pH (11.0), or in the presence of a reducing agent (dithiothreitol), conditions that cause dissociation of the oligomer. AFM imaging of syncollin attached to supported lipid bilayers again revealed doughnut-shaped structures (outer diameter 31 nm, inner diameter 6 nm) protruding 1 nm from the bilayer. Finally, addition of syncollin to liposomes rendered them permeable to the water-soluble fluorescent probe 5(6)-carboxyfluorescein. These results are discussed in relation to the possible physiological role of syncollin.