When 10 strains of lactic acid bacteria were incubated with 5'-hydroxyaverantin (HAVN), a precursor of aflatoxins, seven of them converted HAVN to averufin; the same reaction is found in aflatoxin biosynthesis of aflatoxigenic fungi. These bacteria had a dehydrogenase that catalyzed the reaction from HAVN to 5'-oxoaverantin (OAVN), which was so unstable that it was easily converted to averufin. The enzyme was purified from Lactobacillus brevis IFO 12005. The molecular mass of the enzyme was 100 kDa on gel filtration chromatography and 33 kDa on SDS polyacrylamide gel electrophoresis (SDS-PAGE). The gene encoding the enzyme was cloned and sequenced. The deduced protein consisted of 249 amino acids, and its estimated molecular mass was 25,873, in agreement with that by time of flight mass spectrometry (TOF MS) analysis. Although the deduced amino acid sequence showed about 50% identity to those reported for alcohol dehydrogenases from L. brevis or L. kefir, the commercially available alcohol dehydrogenase from L. kefir did not convert HAVN to OAVN. Aspergillus parasiticus HAVN dehydrogenase showed about 25% identity in amino acid sequence with the dehydrogenase and also with these two alcohol dehydrogenases.