A twice daily application of 2% erythromycin base in hydroalcoholic solution accompanied by once daily use of 0.05% tretinoin (retinoic acid) solution was substantially more effective than tretinoin or erythromycin alone for treatment of inflammatory acne of moderate severity. Therapeutic enhancement by this combination can be attributed to the different modes of action, erythromycin acting chiefly by suppressing Propionibacterium acnes, while tretinoin is comedolytic. In addition, by altering the horny layer barrier, tretinoin doubtless increases the penetration of erythromycin.