The intergastric bacterial flora was studied in 125 patients after different types of ulcer operations and correlated with pH value and histological state of gastric mucosa. Following vagotomy most of the gastric juices were sterile whereas more than 60% of the resected stomach were contaminated with fecal and oropharyngeal flora. After jejunal transposition the proportion of fecal flora was reduced to one-third. There was a good correlation between bacterial overgrowth and pH value. Only 10-20% of the stomachs with pH values lower than 4.5 and more than 80% of the stomachs with pH values above 6 were bacterially contaminated. Intragastric flora was found in 90% of the stomachs with atrophic gastritis. The synergistic role of intragastric bacterial flora in carcinogenesis by deconjugation of the bile acids and production of nitrosamine is emphasized.