The distribution of tissue polypeptide antigen (TPA) was studied in unfixed, methanol-, 95% ethanol-1% acetic acid (EA)-, and formalin-fixed paraffin-embedded sections of all adult human tissues using an indirect immunoperoxidase method. The specific staining patterns were virtually identical in unfixed and alcohol-fixed tissues, but in formalin-fixed tissues this similarity was found only after fixation for up to 24 hr and pretreatment with protease for 15 min. Although prolongation of formalin fixation beyond 48 hr increasingly diminished the TPA reactivity, TPA could still be demonstrated in tissues fixed in formalin for up to 6 months. TPA was found to be a cytoplasmic constituent of almost all adult human duct and cavity lining, simple, and stratified epithelia. TPA was not demonstrated in epidermis, renal proximal convoluted and testicular tubules, basket-like myoepithelial cells, nor in most glandular acini, including hepatocytes and pancreatic acinar cells. The TPA staining was also negative in all non-epithelial tissues, including lymph nodes and bone marrow. The well-defined epithelial distribution and the comparable demonstrability in differently preserved tissues make TPA a useful tool for the identification of cells of epithelial character.