This paper presents the method and results of a parametric study of instability toughness, as measured by the plane-strain K_Ic and plane-stress K_c fracture toughness of a multiaxially stretched acrylic plastic. A large number of computer-controlled tests were conducted with the compact tension specimen under high compliance loading/load control. These tests reveal the significant effects on K_c and K_Ic of a wide range of loading rates, specimen thicknesses, and specimen geometry. A direct and immediate application of the results is the recommendation of a new fracture toughness acceptance test for these materials.