To establish a reliable laboratory assay for quantitating topical anti-inflammatory efficacy, the method of Tonelli et al was modified by employing felt fixed forceps, felt and sharp punch in Wistar rats. Croton oil, applied topically to the rat ear, elicited an acute phlogistic response which was maximal 6 hr after the application. The phlogistic response elicited by a single topical application of croton oil (1, 2, 5 and 10%) was increased in a dose-dependent manner, and croton oil, 5%, induced 63.2% increase of ear weight and was the optimal concentration for the experiment. Using this procedure, the antiphlogistic potencies of two corticoids were assayed under conditions of a blind test. ED50 of betamethasone valerate and diflucortolone valerate was 0.26 mg/ml and 0.0097 mg/ml, respectively, in Wistar rats, and 0.86 mg/ml and 0.016 mg/ml in Sprague Dawley rats, suggesting that diflucortolone valerate has an antiphlogistic potency of 27-56 relative to that seen with betamethasone valerate with minor differences in the strain of the rat. Our method should prove to be a useful assay for rapidly quantitating antiphlogistic activities of topically applied corticoids.