We have reported that LG21 yogurt containing Lactobacillus gasseri OLL 2716 (LG21 yogurt) inhibits the formation of HCl-induced acute gastric lesions through the generation of prostaglandin E₂. This study aimed to determine the role of viable Lactobacillus in the healing of acetic acid-induced chronic gastric ulcer. LG21 yogurt or γ-ray radiated LG21 yogurt was administered orally twice a day for 10 d at a dose of 5 ml/kg. LG21 yogurt significantly accelerated the healing of the ulcer, but γ-ray radiated LG21 yogurt did not. However, both yogurts significantly inhibited HCl-induced gastric erosive lesions and enhanced the generation of gastric mucosal prostaglandin E₂. From the above results, it was found that viable bacteria are needed to accelerate the healing of chronic gastric ulcer, but not to inhibit gastric lesions.