Various methods for measuring percutaneous absorption are discussed with particular reference to glucocorticosteroids. Increasing concentrations of steroids in a given formulation will yield a decreasing percent of amount penetrating and a corresponding proportionate decrease in biologic activity. The formulation can shift the relationships of concentration to penetration and to biologic activity. The use of bioassay methods to detect penetration of antibiotics has been applied to find an active topical antibiotic for treatment of acne vulgaris.