Two human diffuse gastric carcinoma cell lines were established in vitro from xenografted tumours serially passaged in nude mice. Of 12 primary diffuse gastric carcinomas, 7 were successfully xenografted in nude mice (58.3%). Short-term primary cultures were achieved in all the xenografted lines. However, only 2 of the 7 short-term primary cultures were established as long-term cultures (GP202 and GP220). GP202 cells are larger than GP220 cells, show less abundant intercellular junctions at the ultrastructural level and grow in culture as a compact thin monolayer. The GP220 cells grow preferentially in small clusters attached to the monolayer, with a subpopulation of floating cells. Both lines have cells containing small mucin vacuoles in the cytoplasm and cells displaying a typical signet-ring shape. GP202 cells grow as solid tumours in nude mice but GP220 cells do not give rise to tumours. The flow cytometry and karyotype analysis showed aneuploidy in GP202 cells, with many numerical and structural chromosomal abnormalities, and diploidy in GP220 cells, with several structural chromosomal abnormalities. The CDw75 and Tn antigens are more prominently expressed in GP202 cells than in GP220 cells. T antigen is only expressed in GP202 cells, whereas only GP220 cells express EGFR. Sialosyl-Tn is not expressed in either of the cell lines. The gastric cancer cell lines described in this paper represent a valuable addition to the small number of diffuse gastric cancer cell lines currently available and also provide a good model for further in vitro and in vivo studies of gastric carcinogenesis.