The rheological properties of a sunflower protein isolate (SFPI) were studied, and injection-molded pieces were produced. Various mixtures of SFPI (100 parts by weight), glycerol (10-50 parts), water (10-50 parts), and sometimes sodium sulfite (used as reducing agent, 1-10 parts) were studied. Power-law-type modeling of the viscosity as a function of the moisture and glycerol contents described the thermoplastic properties of the proteins under certain conditions. The presence of sodium sulfite in the mixture seemed to improve the flow properties substantially, with an optimum at 4 parts (w/w). Injection-molded objects were then made, and their mechanical properties were tested. The maximum tensile strength (16.1 MPa) was obtained for 4 parts of glycerol (with 100 and 18 parts of protein and water, respectively) and decreased with increasing glycerol content to 10.6 MPa for 22 parts of glycerol. The elastic modulus was between 2.0 and 0.5 GPa, with a strain at break of no more than 1.8%.