The action of the corticoid drugs has been investigated in regard to their antimitotic activity on human skin. For topical studies two 4-mm biopsies were taken from the forearms of each volunteer to allow use of the paired comparison method; a separate control group was used for oral studies. The results indicated that orally administered triamcinolone and topically administered triamcinolone acetonide inhibited mitotic activity of the epidermis. Topically administered 0.2% fluocinolone acetonide proved equally potent, although no effect was seen when 0.025% was used. No mitotic inhibition was observed after topical administration of hydrocortisone acetate. Repeated injections of corticotropin, however, resulted in a significant fall in mitotic activity. The antimitotic activity of the corticosteroids seems to be related to their therapeutic efficacy. This may supplement the vasoconstrictor assay for pertinence to those diseases with increased cell turnover.