A monoclonal antibody to proliferating cell nuclear antigen (PCNA) has been previously shown to be capable of identifying proliferating cells. We investigated proliferative activity in the healing process of acetic acid-induced gastric ulcer by immunohistochemical staining of PCNA and 5-bromo-2-deoxyuridine (BrdU), and the two methods were compared. Cell proliferative activity of regenerated mucosa around ulcers showed continuous acceleration for 42 days, and PCNA-labeled cells had stained nuclei as clearly discernible as those of BrdUlabeled cells. In addition, immunohistochemical staining of PCNA provided reproducible and quantifiable results without the requirement of pretreatment. We conclude that immunohistochemical staining of PCNA may represent a useful technique for analysis of proliferative activity during healing of gastric ulcers.