The production and postsecretory stability of gastrin-releasing peptide (GRP) and the C-terminal part of the GRP precursor were studied in small cell lung cancer cell lines using RIAs developed against these two parts of the precursor. In three otherwise different cell lines (NIC-H345, NIC-H69, and NIC-H510), similar chromatographic patterns of mainly GRP-(18-27) and some GRP-(14-27) along with large fragments of the C-terminal counterpart of the precursor were found to be stored in the cells. In tissue culture medium, gel filtration chromatography indicated that postsecretory limited proteolysis of the GRP precursor fragments occurred. The amount of accumulated immunoreactivity varied among the three cell lines and between the two parts of the precursor. In medium in which only low amounts of GRP immunoreactivity accumulated, the radiolabeled GRP was degraded rapidly. When incubated with plasma, GRP-(14-27) disappeared within a few hours, whereas the C-terminal precursor fragments were stable. It is concluded that the postsecretory stability of peptides excised from the GRP precursor in small cell lung cancer cells varies under tissue culture conditions, but epitopes in the C-terminal part of the precursor are more stable in plasma than the small GRP peptides and, thus, may serve as a better indicator than GRP itself for expression of the GRP precursor in cancer cells.