D-(-)-3-Hydroxybutyrate (3HB) oligomer hydrolase was purified from Paracoccus denitrificans. The enzyme was a monomeric protein with an approximate molecular mass of 31 kDa. The isoelectric point of the enzyme was 5.2. Optimum temperature and pH were 35-40 degrees C and 8.0, respectively. The enzyme activity was not affected by sulfhydryl reagents but strongly inhibited by serine proteinase inhibitors. Both 3HB trimer and 3HB dimer were hydrolyzed by the enzyme, indicating that the enzyme is not 3HB dimer hydrolase but 3HB oligomer hydrolase. para-Nitrophenyl esters of short-chain fatty acids were also hydrolyzed by the enzyme. 3HB dimer was hydrolyzed somewhat faster than 3HB trimer. The level of the enzyme activity was almost constant, irrespective of carbon sources for the bacterial growth and of the cultivation conditions.