Two separate studies evaluated the ability of a combination topical gel consisting of 5% benzoyl peroxide and 1% clindamycin to reduce facial Propionibacterium acnes counts in vivo and to decrease the development of resistant organisms. In the first study, the combination gel was compared with 3 topical formulations of 1% clindamycin phosphate (gel, lotion, and solution) in 80 individuals. After only 1 week of treatment, a 99.7% reduction from baseline in facial P acnes count was obtained with the combination gel. This was significantly greater (P < .001) than the 30%, 56%, or 62% reduction obtained with the clindamycin gel, lotion, or solution, respectively. After 2 weeks of treatment, the reduction from baseline P acnes counts with the combination gel was increased to 99.9%, which was again significantly greater (P < .001) than that with 1% clindamycin alone, regardless of the formulation. The second study compared the combination gel with 1% clindamycin gel in 79 patients with mild to moderate acne. After 4 weeks of treatment, the combination gel was more effective than clindamycin alone in reducing the total P acnes count, consistent with the previous study. By week 12, an increase in the number of resistant bacteria appeared in samples from patients using clindamycin alone, while counts of resistant bacteria remained stable or declined in those using the combination gel.