In our laboratory, we observed that a case of small cell carcinoma of the lung (SCLC) stained with a monoclonal antibody (Dako Corp., Bcl-2-124) against the Bcl-2 protein. To determine how common this reaction was, and whether it was specific for SCLC, we stained formalin-fixed, paraffin-embedded tissues from 23 cases of SCLC and 20 cases of squamous cell carcinoma of the lung for Bcl-2. Fifteen of the 23 SCLC cases stained positively for the Bcl-2 oncogene protein. In contrast, weak staining was observed in only three of the squamous cell carcinomas (chi 2-test; P = 0.001). In addition, four small cell carcinoma cell lines (H69, H146, H209, and WB) were tested by immunohistochemistry and flow cytometry for expression of this protein; all four were positive. In these and three other (H128, H432, and H510) SCLC lines, Northern blots of polyadenylated RNA revealed expression of the 6-kb Bcl-2 mRNA. Moreover, Western blots of extracts from these cell lines revealed the characteristic 26-kd band for Bcl-2 protein. In a single cell line, H82, which has previously been characterized as a variant SCLC, we failed to detect expression of the Bcl-2-specific mRNA and protein. We therefore conclude that most cases of SCLC of the lung express the Bcl-2 oncoprotein, which could play a role in the pathogenesis of this disease.