The effects of the molecular weight of atactic poly(DL-lactide) (PDLLA) on the crystallization behaviour and morphology of isotactic poly(L-lactide) (PLLA) ((M) over bar(v) = 3.9 x 10(5)) were investigated using differential scanning calorimetry and polarizing microscopy. PLLA could crystallize from the melt and form spherulites in the presence of PDLLA whose (M) over bar(v) ranged from 6.5 x 10(3) to 3.0 x 10(5). The size of the spherulites in equimolar blends was larger than that of non-blended PLLA, irrespective of the molecular weight of PDLLA. The spherulite structure became more disordered as the molecular weight of PDLLA increased. The induction period of PLLA crystallization in the blends became longer and the initial rise in overall crystallinity became less prominent as the molecular weight of PDLLA increased. A significant depression of the equilibrium melting temperature (T-m degrees) of PLLA was observed for the blends with PDLLA, irrespective of the molecular weight of PDLLA. The crystallinity of PLLA was higher for the equimolar blend with PDLLA having the lowest molecular weight (6.5 x 10(3)) than for other blends and the nonblended PLLA when they were annealed for 600 min. The time required for saturation in PLLA crystallization was shorter for the blends annealed after quenching from the melt than for those annealed directly from the melt without quenching.