SUMMARYThe dorsal skin of the hairless mouse when ‘stripped’ with sticky tape proved to be a suitable model for a study of the antimitotic effect of betamethasone‐17‐valerate and hydrocortisone. The application of 1%, betamethasone‐17‐valerate and 1%, hydrocortisone creams under occlusive dressings for 24 h produced a three‐ to four‐fold drop in the colcemid‐induced mitotic index as compared to a control application.The antimitotic effect of the betamethasone‐17‐valerate preparation started at or before 5 h and continued for at least 4 days after removal of the active material. It was shown in addition, that 1% betamethasone‐17‐valerate also reduced the number of cells entering DNA synthesis. The antimitotic effect of hydrocortisone was detected at a concentration of 10‐2and that of betamethasone‐17‐valerate was detected at a concentration of 10‐4. It is suggested that the animal model used is a useful one for assaying the potency of