The expression of c-myc oncogene product was studied in 213 cases with gastric carcinoma by an immunoperoxidase method using a monoclonal antibody (MYC-1). Fifty (23.5%) of 213 tumors showed immunoreactivity to MYC-1. The distribution of c-myc-product-positive cells was observed mainly at the marginal area of’the tumor. Excess reactivity to c-myc product occurred more frequently in invasive cancers than in localized cancers, and c-myc production expression in cancer tissue correlated well with peritoneal dissemination. Patients with c-myc-protein-positive tumor had significantly poorer prognosis than those with c-myc-protein-negative tumor in invasive gastric carcinomas, and the c-myc product status correlated well with the recurrence of cancer by peritoneal dissemination. These results suggest that the expression of c-myc gene product might be related to the proliferative activity of gastric carcinoma and serve as a new biologically relevant tumor marker for determining the prognosis DNA-binding activity [10] and turns over with a half-life as short as 20–30 min [11]. These results suggest that the c-myc gene product may have a role in cell cycle control [12]. This work is a report on the retrospective, immuno-histological analysis of gastric carcinoma we performed using a monoclonal antibody (MYC-1) to the c-myc gene product in order to obtain insight into the role of this protein in the progression of gastric carcinoma.