An ethanolic extract of Cranberry exerted a significant antimicrobial effect on Saccharomyces bayanus and Pseudomonas fluorescens. The antimicrobial properties of cranberries were due to a number of factors. First, the low pH (2.6) inhibited many microorganisms per se, and its effect on the dissociation of benzoic acid made the inhibition more drastic. Secondly, after raising the pH to 5.2, cranberry juice still did not support the growth of S. bayanus. Growth did occur at pH 5.2 after 0.3% yeast nitrogen base was added. Thirdly, proanthocyanidins and flavonols were found to be the major microbial inhibitors other than benzoic acid. The results showed that proanthocyanidins provided 21.3% of the inhibition, the flavonols 18.5% and benzoic acid 15.6%.