The neoplasms and gastric mucosa colonization with Helicobacter pylori was studied in 40 stomachs resected for expansive (n = 20) and infiltrative (n = 20) carcinoma using Giemsa staining histological sections, histochemical reactions to oxyreductase, and urease test. In expansive carcinoma of the stomach, H. pylori was identifiable both in the tumour and surrounding gastric mucosa in 70 per cent of cases; infiltrative carcinoma appeared to be associated with gastric mucosa H. pylori in 80 per cent, whereas tumour H. pylori were recordable in 30 per cent of cases, which fact is dependent upon the histological structure of carcinoma. The role H. pylori plays in cancerogenesis is discussed.