Propionibacterium acnes was grown on Eagle's medium for 4-15 days at pH 5.3-7.2 The porphyrin production was measured both by direct fluorometry and by high pressure liquid chromatography (HPLC). The greatest concentration of porphyrins was produced at pH 6.1. Protoporphyrin was the dominant porphyrin species present in the bacteria in all samples. The relative amount of coproporphyrin was greatest at pH 6.7 after 4 days of incubation. In human skin there are local variations in the pH; therefore our findings may be of importance for porphyrin production in acne.