D (−)- and L (+)-lactate dehydrogenases of Lactobacillus plantarum (WSO) are the probable enzyme which catalyze the reduction of the alpha-keto acids (pyruvate, alpha-ketobutyrate, alpha-ketovalerate, and alpha-ketocaprylate) during anaerobic mannitol metabolism. This is indicated by the fact that each of these compounds are substrates for purified D (−)- and L (+)-lactate dehydrogenases. There was no increase in the alpha-ketobutyrate reduction activity when cells were grown on mannitol with alpha-ketobutyrate as an electron acceptor compared to cells grown on glucose. HPLC ion exchange chromatography of cel extracts of L. plantarum grown on mannitol with alpha-ketobutyrate at pH 7·5 and 5·5 revealed no enzymes capable of alpha-ketobutyrate reduction other than lactate dehydrogenases.