The localization of pepsinogens (PG A and PG C) was studied intracellularly in human gastric biopsies embedded in Lowicryl K4M, using affinity-purified antibodies and protein A-gold. The homogeneous secretory granules of the chief cells contained both PG A and PG C, as was proved by serial sections. Identical reaction was also seen in the core of the biphasic mucous neck cell granules, whereas the mantle did not label. The rough endoplasmic reticulum (RER) and Golgi complex of the chief cells and mucous neck cells contained ample label. Transitional cells identified by the presence of granules of both chief cells and mucous neck cells were recognized. This type of mucous neck cell is thought to transform into a chief cell. However, an increase of RER that could explain an increase of the pepsinogen production was not observed. A mixture of these granules was also found in cells morphologically characterized as young parietal cells, suggesting a common precursor for these three cell types. These observations make the transformation from mucous neck to chief cells questionable. Antral gland cells contained only PG C, as was shown in serial section, too.