The putative retinoblastoma gene (Rb) is a tumor suppressor gene which is believed to cause retinoblastomas when both alleles are inactivated, leading to lack of the encoded Mr 110,000-116,000 phosphoprotein. Inactivation of the Rb gene has also been found in several other tumor types, including small cell lung cancer (SCLC). Absence of the 4.7 kilobase mRNA has been found to be frequent in SCLC, and it has been reported that the Rb Mr 110,000-116,000 protein product is always absent, even in tumors expressing Rb mRNA. Using Western blotting technique with a monoclonal antibody directed against the Rb protein, we investigated the expression of the Mr 110,000-116,000 Rb protein in SCLC tumors grown as xenografts in nude mice and/or as cell lines. Rb messenger RNA expression was determined by Northern blotting, and gross structural gene alterations were investigated by Southern blotting. Tumors established from 23 patients were studied. Seven of the tumors did not express Rb protein, whereas expression was detectable in 13. Three tumors were not investigated for protein expression. Only two tumors expressed Rb mRNA without detectable Rb protein expression. Gross DNA alterations were found in four tumors, of which only one expressed Rb mRNA. Our results demonstrated frequent absence of Rb mRNA and protein in SCLC, but apparently normal Rb mRNA and protein were both expressed in more than one-half of the tumors.