Twenty patients with recurrent oral ulceration participated in a placebo-controlled, double-blind trail of Mysteclin syrup (tetracycline hydrochloride and amphotericin) used as a mouthwash. Though a small, consistent improvement occurred with placebo, there was a significant reduction in mean pair scores and numbers of new ulcers recorded daily during the active-treatment periods, the effect lasting for at least four weeks after treatment was stopped. In contrast to topical steroid preparations, Mysteclin syrup is efficacious when begun at any stage of the disorder and is not associated with adverse systemic effects.