Topical formulations of erythromycin and benzoyl peroxide are popular and effective treatments for mild to moderate acne vulgaris. Use of the former is associated with resistance gain in both skin propionibacteria and coagulase-negative staphylococci, whereas use of the latter is not. We evaluated the efficacy of a combination of erythromycin and benzoyl peroxide against a total of 40 erythromycin-sensitive and -resistant strains of Staphylococcus epidermidis and skin propioni- bacteria in vitro. Using the checkerboard technique, five erythromycin resistant strains of Propionibacterium acnes were inhibited synergistically or additively by the combination. Complete mutual indifference was exhibited between the drugs against the remaining 35 strains. However, erythromycin resistant staphylococci and propionibacteria were inhibited by the same concentration of benzoyl peroxide as erythromycin-sensitive strains. These results suggest that, although the combination of erythromycin and benzoyl peroxide is not synergistic against the majority of erythromycin-resistant staphylococci and propionibacteria, the concomitant therapeutic use of both drugs should counteract the selection of erythromycin-resistant variants and reduce the number of pre-existing resistant organisms on the skin of acne patients.