Various enantiomeric isomers, metals salts and molecular sizes of poly(gamma-glutamic acid), gamma-PGA, produced by Bacillus licheniformis CCRC 12826, were prepared and their antifreeze activities were studied by differential scanning calorimetry. The antifreeze activity of gamma-PGA increased as its molecular weight decreased but was indifferent to its D/L-glutamate composition. The antifreeze activity was cation dependent decreasing in the order Mg2+ >> Ca2+ similar to Na+ >> K+ which follows that of inorganic chlorides in that high ionic charge leads to high antifreeze activity. The mechanism by which the cryoprotective effects of gamma-PGA can be explained is still yet to be determined.