A human gastric carcinoma cell line TMK-1 was established in vitro by the soft agar method from SC-6-JCK, a poorly differentiated adenocarcinoma xenotransplanted in nude mice. TMK-1 cells had a doubling time of approximately 35 hr and showed carcinoembryonic antigen (CEA), alpha 1-antitrypsin and secretory component immunoreactivity. Ultrastructurally, the tumor cells were characterized by numerous mitochondria, tubulovesicles and intracytoplasmic canaliculi filled with abundant microvilli. The growth of TMK-1 cells was promoted by 10nM human gastrin (G-17), 2 microM tetragastrin or 2 microM pentagastrin, among which human gastrin showed the most effective growth promotion. Moreover, incorporation of [3H]thymidine into TMK-1 cells was stimulated by gastrin in a dose-dependent manner. The content of cyclic adenosine 3',5'-monophosphate (cAMP) in TMK-1 cells was increased by gastrin treatment but decreased to the control level within 10 min. cAMP-dependent protein kinase was also activated by gastrin administration.