Background: Porphyrins produced by Propionibacterium acnes exhibit an orange-red fluorescence under UVA light. The amount of fluorescence can be estimated by digital fluorescence photography. Objective: We thought that digital fluorescence photography would be a quicker and simpler method than bacteriologic culture to demonstrate depopulation of P acnes in sebaceous follicles. We used benzoyl peroxide to bring about rapid suppression of P acnes. Methods: Benzoyl peroxide 10% was applied twice daily for 7 days to the faces of 9 subjects. Five subjects were untreated controls. Digital fluorescence photographs of cheek and nose, and scrub samples for quantitative recovery of P acnes from the cheek were taken at baseline, day 3, day 7 (end of treatment), and day 16 (regression phase). Results: The effect of benzoyl peroxide against P acnes was clearly demonstrated both by culture and by fluorescence photography after only 3 days. Image analysis of porphyrin fluorescence correlated well with the decrease in P acnes density from scrub cultures. No further decrease was observed at day 7 (end of therapy). Ten days later there was a return to baseline values, although in some subjects these remained lower. Conclusion: Digital fluorescence photography is a reliable, fast, and easy screening technique to demonstrate the suppressive effect of topical antibacterial agents on P acnes. (J Am Acad Dermatol 1999;41:710-6.)