Gastrin-releasing peptide (GRP) is a bombesin-like peptide present in mammalian tissues. Using GRP radioimmunoassay specific for carboxyl-terminal portion,the immunoreactive GRP (IR-GRP) content of 5 fetal lungs, 38 adult lungs, and 131 primary lung tumors was determined. All fetal lung extracts contained IR-GRP ranging from 31-140 ng/g, wet weight. IR-GRP was present in 7-21% of normal adult lungs and lung carcinomas other than small-cell carcinoma; the amount was not very large except in 2 cases of adenocarcinoma, in which 110 and 140 ng/g of IR-GRP were detected. In the cases of small-cell carcinoma, IR-GRP was found in 23 of the 31 cases examined (74%), and 9 (29%) of these contained large amounts of IR-GRP (100-14,000 ng/g). As for carcinoid tumors, IR-GRP was found in 5 of the 12 cases examined (42%) and large amounts of IR-GRP were detected in 2 cases (5100-130,000 ng/g). Immunohistochemically, IR-GRP was found in the neuroendocrine cells of fetal lungs and in the tumor cells of primary lung tumors. When these tissue extracts were examined by bombesin radioimmunoassay that recognizes bombesin but not GRP, they did not contain immunoreactive bombesin, suggesting that IR-GRP in these tissues is more similar to GRP than to bombesin. Sephadex G-50 filtration always revealed 2 peaks of IR-GRP in both fetal lungs and IR-GRP-producing tumors. One was eluted at the position corresponding to that of porcine GRP (Peak 1) and the other, at the position just behind that of porcine GRP (14-27) (Peak 2). In the 5 fetal lungs, Peak 2 comprised more than 83% of the total IR-GRP. In the 12 IR-GRP-producing tumors examined, the ratio of these 2 peaks differed from case to case. IR-GRP, which is present in fetal lung, apparently is often produced by primary lung tumors, especially by small-cell carcinoma and carcinoid tumor, with molecular size heterogeneity.