Various strains of cutaneous micro-organisms were tested in vitro for their survival rates in 0.5 mol/l (8.4% w/v) azelaic acid solution. All bacterial strains exhibited large reductions in viability (at least 40-fold) over a 24 h test period, but little response was noted with Pityrosporum ovale. The bactericidal effect of azelaic acid was reduced considerably in the presence of nutrients. Minimum inhibitory concentrations (MICs) and minimum bactericidal (or fungicidal) concentrations (MBCs) were also determined. MICs varied from 0.03 mol/l to 0.25 mol/l; MBCs were all either 0.25 mol/l or greater.