Despite numerous histochemical, ultrastructural, and immunohistochemical studies, differentiation between malignant epithelial pleural mesothelioma and adenocarcinoma of the lung remains extremely difficult. Although there is general agreement that immunohistochemical methods can aid in this distinction, some studies have produced conflicting results with some of the proposed markers for mesothelioma. To obtain comparable and reproducible results, 19 unequivocal epithelial mesotheliomas and 23 unequivocal primary lung adenocarcinomas were studied by the avidin-biotin-peroxidase complex method on formalin-fixed, paraffin-embedded tissue specimens. Well-characterized, commercially available antibodies to carcinoembryonic antigen (CEA), a high- and low-molecular-weight keratin, vimentin, epithelial membrane antigen, human milk fat globule, Leu-M1, TAG-72 (identified by monoclonal antibody B72.3), beta 1 pregnancy-specific glycoprotein (SP1), human placental lactogen, secretory component (SC), CA19-9, and S-100 protein were used. Twenty-one adenocarcinomas (91.3%) reacted for CEA, 14 (60.9%) for Leu-M1, 14 (60.9%) for SC, nine (39.1%) for CA19-9, and eight (34.8%) for SP1; no mesotheliomas stained for any of these markers. Nineteen adenocarcinomas (82.6%) and one mesothelioma (5.3%) reacted with B72.3. Adenocarcinomas and mesotheliomas did not significantly vary in reaction to the remaining antibodies. None of the antibodies used was specific for mesothelioma, but CEA was the single most useful marker. One of the two adenocarcinomas negative for CEA was positive for TAG-72, Leu-M1, and SC, and the only B72.3-positive mesothelioma was negative for CEA, Leu-M1, SC, CA19-9, and SP1. These findings indicate that greater sensitivity in differentiating mesothelioma and adenocarcinoma can be achieved by immunostaining for both CEA and one or more of the markers TAG-72 (B72.3), Leu-M1, SC (these three have the highest sensitivity and specificity after CEA), CA19-9, and SP1.