With anionic waterborne polyurethane (WPU) as a plasticizer and ethylene glycol diglycidyl ether (EGDE) as a crosslinker, we successfully prepared crosslinked soy protein isolate (SPI) plastics. Anionic WPU was mixed with SPI and EGDE in an aqueous dispersion at room temperature. The mixed aqueous dispersion was cast and cured, and the obtained material was pickled and hot-pressed to produce the crosslinked SPI/WPU sheets. The resulting sheets containing about 60 wt% SPI were characterized with infrared spectroscopy, scanning electron microscopy, atomic force microscopy, dynamic mechanical analysis, and tensile testing, and biodegradation testing of the sheets was performed in a mineral salt medium containing microorganisms. The results revealed that the crosslinked SPI/WPU plastics with EGDE concentrations of 2-4 wt%, possessed high miscibility, good mechanical properties, and water resistivity. In addition, the crosslinked sheets could be biodegraded, and the half-life of the biodegradation for a sheet crosslinked with 3 wt% EGDE was calculated to be less than 1 month. (C) 2004 Wiley Periodicals, Inc.