Poly(lactic acid) (PLA)/soy protein isolate (SPI) blends with and without modifier were melt compounded in a intensive mixer. Tensile strength of PLA/SPI simple blends decreased more than pure PLA because the dispersed SPI granules acted as the stress concentration; tensile strength was improved by loading of sodium bisulfite (NaHSO3). At 0.5 wt % NaHSO3 loading, the blend gave 43.9 MPa of tensile strength. Blends containing both methylene diphenyl diisocyanate (MDI) and NaHSO3 displayed a significant improvement in tensile strength, which increased by 38% at 4 wt % MDI, compared with the PLA/SPI/NaHSO3 blend. The scanning electron microscopic observation demonstrated that incorporating NaHSO3 and MDI improved compatibility between PLA and SPI. The differential scanning calorimetry indicated that SPI induced and accelerated cold crystallization of PLA in the blends, which was also affected by the presence of NaHSO3 and MDI. The PLA/SPI blend containing both NaHSO3 and MDI displayed higher water resistance than the PLA/SPI/NaHSO3 blend. (C) 2009 Wiley Periodicals, Inc. J AppI Polym Sci 114: 754-759, 2009