Two unique human signet ring cell gastric carcinoma cell lines (designated HSC-39 and HSC-40A) were established in vitro from the ascites of a 54-year-old male patient. Both cell lines were biologically quite similar, grew in vitro in suspension with a population doubling time of 28-30 h, and had cytological features of mucinous epithelial tumor cells. They formed colonies in soft agar, with a cloning efficiency of 0.8-1.0%. Ultrastructurally, numerous granules were observed in the cytoplasm, suggesting secretory activity. The frequent presence of desmosome and the tight junction at the cell boundary certifies the epithelial origin of the lines. Immunocytochemistry and radioimmunoassay showed production of tumor marker antigens (carcinoembryonic antigen, CA 19-9, and sialyl-Lex-i) and gastrin in both lines. These lines were transplantable in athymic BALB/c nude mice. The histopathology of each line growing in athymic BALB/c nude mice was similar to that of the original tumor. The karyotype of the cells was highly aberrant with structural and numerical changes. The presence of numerous double minute chromosomes and loss of the 13 chromosome and Y-chromosome characterize these lines. In addition, the amplified c-myc oncogene (16-32-fold) was found in both cell lines and original ascitic tumor cells. Overexpression of the c-myc mRNA was noted. These cell lines may be a useful tool, providing both in vivo and in vitro systems for further studies of the biology and therapy of human signet ring cell (or Borrmann's type IV carcinoma) gastric carcinoma.