Nine lung small-cell carcinoma (SCC) cell lines and 9 lung non-SCC cell lines were examined for structural changes of the retinoblastoma (RB) gene as well as its expression using a complementary DNA probe. The RB protein product was investigated using an anti-RB antibody which we produced. Although homozygosity or hemizygosity of the RB gene was suggested in 8 of 9 SCCs and one of 2 large cell carcinomas (LCCs) by Southern blot analysis using an RB cDNA probe and polymorphic DNA markers for chromosome 13, no obvious structural changes of the RB gene were detected in these 18 cell lines. However, RB transcripts were either markedly reduced in quantity or abnormal in length in 3 of 9 SCCs. The specific 115 kD protein was not immunoprecipitated by the anti-RB antibody in all 9 SCCs with either normal or abnormal size RB mRNA. Three of 4 adenocarcinomas (AdCs), all 3 squamous cell carcinomas, and one of 2 LCCs expressed normal size RB mRNA, and the 115 kD protein was immunoprecipitated by the anti-RB antibody. The 115 kD protein was also absent in one of 2 LCCs with shortened RB mRNA and in one of 4 AdCs with low level of RB mRNA expression. These results strongly suggest that inactivation of the RB gene might be involved in the development of lung cancers, especially of SCCs.