['Antibodies against the thyroid transcription factor-1 (TTF-1) and the surfactant proteins A and B (SPA, SPB) were compared as paraffin-reactive immunohistochemical markers for non-small cell carcinomas of pulmonary origin.', 'We studied 138 carcinomas of pulmonary origin (98 adenocarcinomas, 20 non-neuroendocrine large cell carcinomas, 20 squamous cell carcinomas) and a total of 276 extrapulmonary carcinomas of various primary origins. Using the monoclonal antibody 8G7G3/1, TTF-1 was detectable in 75% of non-mucinous pulmonary adenocarcinomas and in 40% of large cell carcinomas but in only 10% of mucinous adenocarcinomas and not in squamous cell carcinomas. In contrast, both SPA and SPB were positive in only 45% of pulmonary adenocarcinomas and in 10% and in 5% of the large cell carcinomas, respectively. TTF-1 had a specificity of 0.98 for pulmonary carcinomas as 5/7 thyroid carcinomas were the only TTF-1-positive extrapulmonary tumours. Anti-SPB and anti-SPA had specificities of 1. 00 and 0.97, respectively.'] Is thyroid transcription factor-1 the superior immunohistochemical marker for pulmonary adenocarcinomas and large cell carcinomas compared to surfactant proteins A and B?