SUMMARYIn the oxazolone‐induced delayed hypersensitivity inflammation in the rat ear, betamethasone 17‐valerate, in contrast to other topical corticosteroids, is incapable of suppressing oedema. When given in combination with triamcinolone acetonide, betamethasone 17‐valerate competitively anta‐gonizes the anti‐inflammatory action of the active steroid. When tested in the mouse, betamethasone 17‐valerate behaved as an anti‐inflammatory agent 15 and 80 times as potent as betamethasone and hydrocortisone respectively. In anin vitrolymphocyte culture system in which preincubation with corticosteroids prevents subsequent phytohaemagglutinin induced DNA synthesis, betamethasone 17‐valerate was less active than even hydrocortisone when rat lymph node cells were used, but with human cell preparations it was more potent than either hydrocortisone or betamethasone.Betamethasone 17‐valerate behaves uniquely in the rat as an anticorticosteroid; in mouse and in man the compound behaves as a norm