The distinction between malignant epithelioid pleural mesothelioma (MEPM) and peripheral adenocarcinoma of the lung with pleural invasion (PAL) continues to represent a diagnostic challenge in selected cases. In order to provide comparative data on histologic, histochemical, and immunohistochemical features of these neoplasms, we analyzed 51 ultrastructurally categorized MEPMs and 52 PALs with the periodic acid-Schiff-diastase (PAS-D), mucicarmine, and colloidal iron stains, and a panel of immunohistologic reagents. Antibodies to cytokeratin, vimentin, epithelial membrane antigen (EMA), carcinoembryonic antigen (CEA), Leu M1, the B72.3 antigen, blood group isoantigens (BGI), placental alkaline phosphatase, amylase, S100 protein, and Clara cell antigen were used, as applied to paraffin sections with the avidin-biotin-peroxidase complex technique. Ultrastructural studies revealed long, branching microvilli in MEPM cells in all cases, with length-to-diameter ratios (LDR) of 10:1 or more. In contrast, PAL manifested short, nonbranching microvilli with LDR of 8:1 or less. Reactivity with PAS-D and mucicarmine stains was strictly confined to PAL, and hyaluronidase-sensitive colloidal iron-positivity was restricted to MEPM. However, only 63% and 41% of these respective neoplasms demonstrated such histochemical reactivity. Immunohistologic results correlated well with electron microscopic classification. All MEPMs and PALs were reactive for cytokeratin; in addition, the majority of tumors in each group expressed EMA, and a minority were reactive for vimentin. In adenocarcinomas of the lung, Leu M1 was observed in all cases, CEA was apparent in 96%, B72.3 labeled 84%, and BGI were present in 67%; all PALs expressed at least two of these determinants, but none was seen in any mesothelioma. The other markers included in this study also were observed in some PAL cases, but not in MEPM. These findings suggest that immunohistology parallels electron microscopy in efficacy in the diagnostic separation of MEPM and PAL. Using antibodies to Leu M1, CEA, and the B72.3 antigen, reactivity for at least two of these three markers appears to exclude a diagnosis of pleural mesothelioma. The other glycoproteinaceous, oncoplacentofetal, and cytoplasmic antigens we studied can be used to reinforce such a determination, since their distribution is confined to adenocarcinomas.