Mucosa-related bacteria, intra-epithelial lymphocytes and intra-epithelial polymorphonuclear leucocytes have been studied in 35 patients with duodenal ulceration, 27 patients with gastric ulceration and eight control subjects with normal gastro-duodenal mucosa. Mucosa-related bacteria were found in approximately 80 per cent of peptic ulcer patients and rarely in controls. The bacteria were most numerous at the sites of active chronic gastritis. There was a positive correlation between the number of bacteria and the number of intra-epithelial polymorphonuclear leucocytes. There was no correlation between the peripheral blood white cell count and the number of intra-epithelial polymorphonuclear leucocytes. The number of intra-epithelial lymphocytes was increased in peptic ulceration.