(radioimmunoassay/immunohistochemistry/peptide hormones/high-

The presence of immunoreactive bombesin in a human lung small-cell carcinoma grown in nude mice was estab- lished by several criteria: (i) Radioimmunoassay of tissue extracts for bombesin revealed approximately 6.5 pmol/g of tissue; (ii) bombesin was found in 12-14% of the tumor cells by immuno- histochemical localization; (iii) gel filtration of small-cell carcinoma extract on Sephadex G-75 and Bio-Gel P-4 gave only a single peak of immunoreactivity, which occurred at the elution volume of bom- besin; and (iv) reverse-phase HPLC of acid-solubilized extracts separated the immunoreactive'material into three discrete peaks, one of which eluted with a retention time identical to that of syn- thetic bombesin. The presence of bombesin may represent the ectopic expression of this peptide in small-cell carcinoma, because immunoreactive bombesin was found in human fetal and neonatal lung but apparently not in adult lung tissue (Wharton, J., Polak, J. M., Bloom, S. R., Ghatei, M. A., Solcia, E., Brown, M. R. & Pearse, A. G. E. (1978) Nature (London) 273, 769-770). The im- munoreactive bombesin previously found in mammalian tissues is considerably larger than amphibian bombesin; these data sub- stantiate the presence of a mammalian form of bombesin in a hu- man tumor that may have a structure similar to that of the am- phibian peptide.