IVarious preparations of dried unripe plantain banana were found to be anti-ulcerogenic against aspirin-induced ulceration in the rat and were effective both as a prophylactic treatment and in healing ulcers already induced by aspirin. Ripe fruit bananas were inactive. 2 The active factor(s) were water soluble and were concentrated by extraction to approximately three hundred times that in the dried banana powder. 3 The anti-ulcerogenic action of banana preparations appears to be due to their ability to stimulate the growth of gastric mucosa. 4 Aluminium hydroxide, cimetidine, prostaglandin E2, N6, 02-dibutyryl adenosine 3',5' cyclic monophosphate but not 5 -hydroxytryptamine were also anti-ulcerogenic when used prophylactically in rats but were ineffective in healing ulcers already formed by aspirin. These substances did not stimulate the growth of gastric mucosa.