A human gastric cancer cell line, STKM-1, was established from the malignant cells in pleural effusion of a 41-year-old female patient. The primary gastric cancer revealed histologically a poorly differentiated adenocarcinoma. The cells have been cultured with RPMI-1640 medium supplemented with 10% fetal bovine serum and grew as monolayers following a doubling time of 31.4 hour at passage 30. The mode of chromosome number was 52. The STKM-1 cell was tumorigenic in nude mice. The STKM-1 cell cultured in vitro secreted CA19-9, into the medium as a tumor marker. Cells in tumors grown in nude mice were immunohistochemically recognized positively by anti-CA19-9 antibody. The STKM-1 will provide a useful information to clarify the mechanism of CA19-9 secretion.