The proliferative activity of human gastric carcinoma was measured by means of in vitro incorporation of the thymidine analogue, 5-bromo-2'-deoxyuridine (BrdU), into the newly-synthesized DNA of fresh tumors and immunohistochemical staining of proliferating cell nuclear antigen (PCNA) using avidin-biotin peroxidase method. Eighty-two cases of surgically resected human gastric carcinomas consisting of 18 various histologic types were subjected to study. The mean BrdU labelling index (LI) and PCNA LI were 22.9% and 39.1%, respectively. The correlation between BrdU LI and PCNA LI was statistically significant (correlation coefficient mu = 0.61334, p = 0.0001). We concluded that immunohistochemical staining for PCNA may become a practical method instead of in vitro or in vivo BrdU labeling to assess the proliferation fraction of the gastric cancer patient.