Cryostat sections of normal human gastric mucosa from the body were treated with monospecific heteroimmunsera, which were directed against the organspecific esterase-antigen VI A. Using fluorescent antiglobulin immunsera it could be demonstrated according to the gastric histology, that the antigen IV A is localized in the mucosa secreting surface epithelial cells. The specificity of the immunohistochemical assay has been proved by absorption studies.