Surgeons have an ingrained suspicion of foreign prostheses implanted into the body and it is a sound principle to avoid their use whenever possible. Unfortunately, in the adequate relief of pulmonary outflow tract obstruction in the tetralogy of Fallot a prosthesis is often necessary.8 This prosthesis has usually been formed of compressed polyvinyl alcohol (Ivalon) sponge. It seemed worthwhile, therefore, to evaluate the reaction to this material when used to replace a portion of the anterior wall of the right ventricle. Method The commercially available Ivalon * was used. It was rinsed in running water according to the instructions and sterilized by autoclaving while immersed in water. Both compressed and uncompressed Ivalon were used. The compressed sponge was so formed by compressing 1 cm. slices of Ivalon in a metal press down to 2 mm. and then autoclaving it in this form under water. The patches used in animals had