Antibodies to the pulmonary epithelial cell-specific proteins surfactant proteins A and B (SP-A and SP-B) and to thyroid transcription factor-1 (TTF-1), a homeodomain nuclear transcription protein, were used as immunohistochemical markers to asses their ability to distinguish primary pulmonary non-small cell carcinomas (n = 57) from carcinomas of the breast (n = 51). SP-A, SP-B, and TTF-1 were detected in 49%, 53%, and 63% of non-small cell carcinomas, respectively. These three antibodies stained pulmonary adenocarcinomas in 54%, 63% and 76% of specimens, respectively. Squamous cell carcinomas rarely stained using these markers. Antibodies to SP-B and TTF-1 never stained any of the 51 breast carcinomas, whereas four of these tumors stained for SP-A. To better define the potential diagnostic value of these antibodies, 13 breast carcinomas metastatic to the lung were studied. Of the three antibodies tested, only TTF-1 seemed useful, because none of the 13 metastatic tumors showed immunoreactivity to this antibody, whereas six specimens (46%) showed reactivity for both SP-A and SP-B. To emphasize further the potential usefulness of antibodies to TTF-1, sections of adenocarcinomas of the colon (n = 18) and prostate (n = 9), renal cell carcinomas (n = 8), and epithelioid mesotheliomas (n = 4) were evaluated; none was positive. Only one of 66 gastric and one of eight endometrial adenocarcinomas showed focal positivity. These results demonstrate the usefulness of immunodetection of a pulmonary cell selective transcription protein (TTF-1) in the diagnosis of pulmonary adenocarcinoma, readily distinguishing breast carcinomas from primary pulmonary adenocarcinomas. In contrast, staining for SP-A and SP-B is of limited value, because there is an unacceptably high rate of cross-reactivity between breast carcinomas metastatic to the lung and primary pulmonary carcinomas. The latter finding illustrates and supports the fact that tumor marker phenotypes might differ in primary and secondary tissue sites.