Ultrastructural immunostaining of carbonic anhydrase in gastric parietal cells was accomplished with the immunoglobulin-peroxidase bridge procedure applied to cryostat sections of fixed guinea-pig stomach prior to dehydration and embedment. Of a variety of fixatives tested, only freshly prepared paraformaldehyde buffered with calcium acetate provided both immunostaining and adequate preservation of ultrastructural morphology. Delipidization or exposure of specimens to detergent prior to staining enhanced the intensity of the immunostaining and increased the sensitivity of the method. Increased diaminobenzidine concentration in the peroxidase substrate appeared also to intensify the densification at the reactive site. Carbonic anhydrase was localized ultrastructurally with this pre-embedment immunobridge procedure in the hyaloplasm of gastric parietal cells and less consistently in the superficial surface epithelium. The basal portion of the parietal cells stained more intensely than the apical region and immunoreactivity appeared concentrated at the plasmalemma and around mitochondria.