Five commercially available corticosteroid creams were compared with respect to their thinning action on mouse tail epidermis. The greatest effects were produced by Synalar and Sintisone; hydrocortisone had the least action. In addition it was shown that the epidermal thickening induced by vitamin A could be significantly reduced by Ledercort cream. The thinning action of the fluorinated steroids on the epidermis is an important property of these agents when used for the treatment of dermatoses having a hyperactive epidermis. The mouse tail test is considered to be a useful screening test for new topical applications of these substances.