Blends of atactic poly[(R,S)-3-hydroxybutyrate], a-PHB, with poly(epsilon-caprolactone), PCL, and with poly(L-lactic acid), PLLA, were obtained in the form of compression-molded films. The phase behavior of the blends was different: a-PHB and PCL were totally immiscible, whereas a-PHB/PLLA blends were miscible over the whole composition range. Biodegradation experiments were carried out on the blends and on the plain polymers in a buffered solution of PHB-depolymerase A from Pseudomonas lemoignei (Tris-HCl, pH 8, T = 37 degrees C). None of the pure blend components (a-PHB, PCL, PLLA) showed any weight loss upon enzyme exposure. Conversely, both a-PHB/PCL and a-PHB/PLLA blends biodegraded. Analysis of the biodegradation products and of blend composition changes during biodegradation demonstrated that in both blends only the a-PHB component undergoes enzymatic hydrolysis. The results support the hypothesis that the crystalline polyester blended with a-PHB promotes a-PHB enzymatic hydrolysis by providing stable binding sites to the enzyme. The dependence of biodegradation rate on blend composition is different in (immiscible) a-PHB/PCL and (miscible) a-PHB/PLLA blends and can be explained in terms of different phase behavior and morphology.