Wheat gluten (WG) was incorporated into polycaprolactone (PCL) (up to 50% w/w) as a filler to form a biodegradable polymer composite. A microscopic examination showed a well-dispersed particle-matrix system. The composite was evaluated for its tensile properties. The tensile strength of the composite decreased linearly with increasing WG content from 20 (0%, WG) to 6 MPa (50% WG). However, the reduction of the tensile strength did not fit the Nicolais-Narkis model, and this indicated that some adhesion between WG and PCL occurred. High elongation (>900%) was observed in PCL-WG composites with up to 20% WG; it decreased to 400% with 35% WG and finally to less than 100% with 40-50% WG. There was a particle-induced transition at a calculated critical volume of 0.3 corresponding to 30% WG by weight with respect to PCL. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110: 2218-2226, 2008