Human gastric cancer was transplanted into the peritoneum of nude mice, and the progress of invasion and growth of cancer were investigated. Serial transplantations succeeded in 4 strains of human gastric cancer and one strain of canine gastric cancer induced by N-thyl-N-nitro-N-nitrosoguanidine (ENNG). The five strains grew subcutaneously in nude mice, and both single strain and mixed strain were transplanted into the mouse peritoneal cavity by a surgical procedure. In the single strain, cancer cells demonstrated mucosal and/or submucosal invasion in the gastrointestinal tract. In the mucosal layer, cancer permeation into the lymphatic duct was verified. In the histological examination, each strain of the mixed ones grew back to back with no interferance, showing front formation. The human strain and canine strain co-existed in the mouse. A human strain of poorly differentiated adenocarcinoma showed hematogenous metastasis to the liver. This is the first report that the invasion as one of biological characteristics of the primary human gastric cancer was clearly demonstrated, and also, another important biological characteristics i.e., the hematogenous liver metastasis was manifested in the mixed strain.