Active butyrate kinase (Buk) and phosphotransbutyrylase (Ptb) were purified in three steps: ammonium sulfate precipitation, hydrophobic chromatography on phenyl-Sepharose and affinity chromatography on Matrex Red A from recombinant Escherichia coli K2006 (pJC7). They were then successfully exploited for in vitro synthesis of 3-hydroxybutyryl-CoA (3HBCoA), 4-hydroxybutyryl-CoA (4HBCoA), 4-hydroxyvaleryl-CoA (4HVCoA) and poly(hydroxyalkanoic acid) (PHA). In addition, the ability of the PHA synthase of Chromatium vinosum, PhaEC(Cv), to use these CoA thioesters was evaluated. Combination of Buk and Ptb with PhaEC(Cv) established a new system for in vitro synthesis of poly(3-hydroxybutyric acid) [poly(3HB)]. In this system, 3-hydroxybutyric acid was converted to 3HBCoA by Buk and Ptb at the expense of ATP. Formation of 3HBCoA was further driven by the polymerization of 3HBCoA molecules to poly(3HB) by PHA synthase, and the released CoA was recycled by Ptb. This system therefore also ensured the regeneration of CoA. With ATP as the energy supply, which was hydrolyzed to ADP and phosphate, 2.6 mg poly(3HB) was obtained from a 1-ml reaction mixture containing 7.6 mg 3-hydroxybutyrate at the beginning. Studies showed that Ptb and PHA synthase were the rate-limiting steps in this system, and initial CoA concentrations ranging from 1 to 7 mM did not inhibit poly(3HB) synthesis. Synthesis of various polyesters of 3HB and 4HB with this system was also tested, and copolyesters containing 4HB of 1-46 mol % were obtained.