Soy protein isolate, soy fiber, and cornstarch (0-40% polyether polyol) were incorporated into a flexible polyurethane foam formulation. Stress-strain curves of the control foam and foams containing 10-20% biomass material exhibit a considerable plateau stress region but not for foams extended with 30-40% biomass materials. An increase in biomass material percentage increases foam density. An increase in initial water content decreases foam density. Foams extended with 30% soy protein isolate, as well as foams extended with 30% soy fiber, have notably greater resilience values than all other extended foams. The comfort factor increases with increasing percentage of biomass material in foam formulation. Foams containing 10-40% biomass materials display significantly lower values in compression-set than the control foam.