The miscibility of binary blends of bacterial poly[(R)-3-hydroxybutyric acid] {P[(R)-3HB]} (M(W)=650000) with poly[(S)-lactic acid] {P[(S)-LA]} of various molecular weights (M(W)=9900-530000) have been studied by means of differential scanning calorimetry (d.s.c.). D.s.c. analysis revealed that the structure of P[(R)-3HB]/P[(S)-LA] blends was strongly dependent on the molecular weight of the P[(S)-LA] component. The blends of P[(R)-3HB] with P[(S)-LA] of high M(W) values over 20000 showed two phases in the melt at 200 degrees C, while the blends of P[(R)-3HB] with P[(S)-LA] of low M(W) values below 18000 were miscible in the melt over the whole composition range. On the basis of the relationship between the miscibility of blends and the molecular weight of the P[(S)-LA] component, the difference in the solubility parameters delta(1) and delta(2) of the blend components in the Flory-Huggins equation was estimated to be 0.34 (J cm(-3))(1/2).