Small cell lung carcinoma (SCLC) represents about 25% of all lung cancers. Human SCLC shows neuroendocrine features such as the production of neural peptide hormones, marker enzymes and neurosecretory granules, and the expression of neural cell adhesion molecules (NCAMs). Although SCLC is sensitive to both chemotherapy and radiation, prognosis remains poor due to the appearance of post-treatment chemo- and radioresistant variants. Monoclonal antibodies (MAbs) have been developed that bind to SCLC tumor antigens. We have used similar technology to define another SCLC marker designated gP94/115. The MAb CR101 binds to a highly glycosylated, cell-surface antigen associated with SCLC. In vitro expression of the antigen appears to be restricted to cell lines of SCLC origin. Enzymatic removal of the sugars resolves the antigen into two proteins of 94 and 115 kD by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Fluorescence-activated cell sorting (FACS) analysis confirms the antibody's specificity. These results indicate that CR101 may recognize a novel protein expressed by SCLC.