Lactobacillus acidophilus has been found to inhibit Candida albicans when grown on MRS agar plates. Attempts to isolate an active factor responsible for this inhibition from liquid culture and agar plates were not successful. The addition of sodium thiocyanate to the agar was found to increase the inhibition offered by the lactobacillus. The results indicate that hydrogen peroxide produced by the lactobacillus is being used to convert the thiocyanate to hypothiocyanate which is more toxic. The involvement of a lactobacillus peroxidase in this conversion is postulated.