We have determined the ultrastructural localization of pepsinogen II (PG II) in aldehyde-osmium fixed, resin-embedded ultrathin sections of human gastric fundic and pyloric gland mucosa and duodenum using the protein A immunogold technique and antiserum specific for PG II. Chief cells contained homogeneous, finely granular secretory granules that were uniformly PG II-positive. In contrast, mucous neck, pyloric gland, and Brunner's gland cells contained secretory granules consisting of an eccentrically placed dense core surrounded by a clear matrix; PG II immunoreactivity was concentrated over the dense core of these biphasic granules. Pyloric gland cells also contained monophasic secretory granules having the same electron density, texture, and PG II immunoreactivity as the nucleoids of biphasic granules and clear vesicular granules that were PG II-negative. Vesicular granules also were found in Brunner's gland cells. The Golgi region of pyloric gland cells contained small PG II-positive monophasic granules adjacent to small PG II-negative vesicular granules. This observation suggests that biphasic granules may form in the Golgi region of pyloric gland cells by fusion of PG II-containing monophasic granules and PG II-unreactive vesicular granules.