Ten infant thymuses and 13 primary thymic tumors obtained from archived paraffin-embedded tissue were examined for the presence of tissue blood group O antigen (H), peanut agglutinin receptor antigen (PNA-r), Saphora japonica agglutinin receptor antigen (SJA-r), carcinoembryonic antigen (CEA), cytokeratin (CK), and epithelial membrane antigen (EMA). In the thymuses studied, Hassall's corpuscles contained abundant immunoreactive CK, PNA-r, and H antigens, whereas CEA, SJA-r, and EMA were present focally in Hassall's corpuscles. Immunoreactive CK, PNA-r, and CEA were demonstrated focally in the subcapsular region, cortical nurse cells, and subcapsular-perivascular monocytic cells, respectively. PNA-r was present in all 12 epithelial type tumors, including all eight thymomas. CEA was present in nine tumors, including six thymomas. Six thymomas contained H antigen and SJA-r; five continued CK and EMA. SJA-r and EMA were also present in one carcinoid tumor of thymic origin. In epithelial thymomas, the antigens stained nests of epithelial cells resembling the pattern of staining in Hassall's corpuscles. Membrane staining of spindle cells of both spindle cell and epithelial thymomas was less intense than staining of epithelial type cells.