Various 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. The active factor(s) were water soluble and were concentrated by extraction to approximately three hundred times that in the dried banana powder. The anti-ulcerogenic action of banana preparations appears to be due to their ability to stimulate the growth of gastric mucosa. Aluminium hydroxide, cimetidine, prostaglandin E2, N6, O2-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.