During our research into the pathogenesis of Vibrio parahaemolyticus, we noticed that the concentration of serum added to the tissue culture medium (Dulbecco's modified Eagle's medium: DMEM) greatly affected its growth. Using gel filtration column chromatography, we clearly demonstrated that serum contains not only a bacterial growth inhibitor (BGI) but also a bacterial growth enhancer (BGE) for Vibrio parahaemolyticus. Our data indicate that the BGI is transferrin, whereas the BGE seems to be an undescribed small molecule (molecular weight of 1,000-3,000 Da) and is associated with magnesium and molybdenum ions. BGE activity was not decreased by heat treatment (at 60 or 100A degrees C for 30 min) and affected the growth rate of a wide range of Gram-negative and Gram-positive bacteria. The addition of EDTA into DMEM lowered the growth rate, whereas the addition of BGE restored the growth activity. This study suggests that sera contain a previously undescribed small BGE molecule.