Healing gastric ulcers were examined immunohistochemically for the presence of myofibroblasts containing actin microfilaments. Twenty five surgical specimens of the gastric ulcer corresponding to the initial healing stage and the proliferative healing stage, and 30 surgical specimens of the acetic acid-induced ulcers in rats at 3, 8 (initial healing stage), and 15 (proliferative healing stage) days after ulcer induction were fixed and cut into 4-micron sections, which were then treated with anti-actin serum, peroxidase-antiperoxidase and incubated for the localization of actin. Controls were prepared using non-immune serum or preabsorbed immune serum. Actin-positive fibroblasts were seen at the edge and the floor of the ulcer in the initial healing stage, but not in the edge of the ulcer in the proliferative healing stage. Such cells may be responsible for the contraction of the ulcer caliver observed clinically in the initial healing stage of the gastric ulcer.