Secretory granule-enriched fractions prepared from isolated rat islets of Langerhans, previously labeled in culture for 18 h with [3H]leucine, have been lysed and separated into pH 5.4 soluble and insoluble fractions by zonal sucrose gradient centrifugation. A high proportion of both labeled and immunoreactive rat insulins I and II were recovered in the insoluble granule core fraction in the expected ratio of approximately 60/40, respectively. Essentially equivalent amounts of the rat C-peptides on a molar basis were recovered in the granule supernatant fractions. The proportion of labeled proinsulin in the granule core fraction was less than 2% relative to insulin, while the soluble fraction contained about 8%, which probably arose mainly from disrupted proinsulin-rich noncrystalline prosecretory vesicles. Electron microscopic examination of the granule core fraction revealed large numbers of well preserved crystalline cores exhibiting typical dimensions and regular internal spacings of normal mature rat beta-granule inclusions. These results provide direct biochemical evidence that the beta-granules are nonuniform in composition with the insulin contained mainly in a crystalline state in the electron-dense central inclusions while the C-peptide is dissolved in the fluid bathing the crystalline hormone. The significance of this structural organization of the beta-granule is discussed.