A monoclonal antibody against proliferating cell nuclear antigen (PCNA), PC10, was applied to 96 gastrectomy specimens obtained from patients with gastric cancer. Resected tissue was routinely processed and stained immunohistochemically with PC10. One thousand tumor cells were counted and the ratio of the number of stained cells to total tumor cells was calculated as the PCNA index, which ranged from 13.8% to 88.8% with a mean of 61.4%. Although the PCNA index did not correlate with pathological variables including histological type, lymph node metastasis, growth pattern and venous invasion, it was significantly lower in mucosal carcinomas compared with submucosal and advanced carcinomas.