One-hundred and sixty-four patients with gastric carcinomas, who underwent gastrectomy during 1979-1985, were studied. The nuclear DNA contents were measured by cytofluorometry, and immunohistochemical study on the expression of c-erbB-2 protein was performed using a monoclonal antibody against the c-erbB-2 oncogene product. Furthermore immunohistochemical detection of proliferating cell nuclear antigen (PCNA) was performed using a monoclonal antibody against the PCNA. The patients with aneuploid tumor had a significantly worse prognosis than those with diploid tumor. The group with positive staining for c-erbB-2 had a significantly worse prognosis than negative one. The higher group of PCNA indices had a worse prognosis. The patients with tumor showing both aneuploid and positive staining for c-erbB-2 tissue status and PCNA indices. But no correlation was formed between c-erbB-2 tissue status, PCNA indices and DNA contents. From these results, it can be concluded that DNA ploidy, c-erbB-2 protein, and PCNA may reflect the malignant potential of gastric carcinoma.