SummaryCiirrently available materials for mechanical heart systems are surveyed, particularly m-ith respect, to long-term implantation, from the viewpoints of availability and workability, mechanical and biological durability, and compatibilit'y with blood and tissue. Some materials have one or more desirable physical properties, btit none offers perfect blood and tissue compatibility. Medical grade Silastic, considered best, for blood handling parts, has produced promising results in auxiliary ventricle patency exneriments. The insufficient tensile stiength of Silastic should be increased, and it may be possible to accomplish this while maintaining the present resilience, elongation, and inertness. No long-term data on thrombus formation are available, but clot-retarding quality is essential and may perhaps be obtained by incorporating heparin. Developments in polymer chemistry may also lead to other surfaces with the requisite mechanical properties with blood.