Previously, high levels of gastrin-releasing peptide and its mRNA were detected in classic small cell lung cancer cell lines. Here the ability of lung cancer cell lines to synthesize neuromedin B (NMB), a structurally similar mammalian bombesin-like peptide, was investigated. By radioimmunoassay, NMB (0.1-0.7 pmol/mg of protein) was detected in 23 of 33 lung cancer cell lines. In contrast, gastrin-releasing peptide (0.1-12.9 pmol/mg of protein) was detected in 16 of 32 cell lines. Using gel filtration and high pressure liquid chromatography techniques, the main peak of immunoreactive NMB coeluted with synthetic NMB. By Northern analysis, a 0.8-kilobase mRNA species was present, using poly(A) mRNA derived from two of three lung cancer cell lines. Using a more sensitive S1 nuclease protection assay, NMB mRNA was present in most of the 15 lung cancer cell lines examined. These data suggest that NMB may be a regulatory peptide in lung cancer.