We wished to assess the antigenic expression of primary lung tumors diagnosed as either carcinosarcoma or sarcoma in order to determine whether this information would be useful in distinguishing the two. We therefore immunohistochemically analyzed six pulmonary carcinosarcomas and five primary lung sarcomas for the presence of carcinoembryonic antigen (CEA), S100 protein, cytokeratin and vimentin using commercially available monoclonal and polyclonal antibodies on formalin fixed tissues. Six of six carcinosarcomas stained positively for cytokeratin while none of the sarcomas stained. In three carcinosarcomas both the carcinomatous and sarcomatous areas were positive while in three only the carcinomatous areas were positive. CEA staining was present in five carcinosarcomas and absent in all the sarcomas. CEA positivity was strong and not confined to those tumors with obvious gland formation. Staining for S100 protein was positive in two carcinosarcomas but only in those areas showing chondroid differentiation. Immunohistochemical staining for vimentin using two different monoclonal antibodies gave inconsistent results. We conclude that in differentiating between a carcinosarcoma and a sarcoma of the lung, immunohistochemical staining for both cytokeratin and CEA are useful with cytokeratin marginally preferable. The data indicate that carcinosarcoma of the lung, like that of the upper aerodigestive tract, expresses antigens suggesting both epithelial and mesenchymal differentiation.