SINCE the first demonstration that silicone rubber1 could be used as an implantable carrier for sustained delivery of low molecular weight compounds in animal tissues, various drug delivery systems have been developed. But except for the reports of Davis2,3 and Girnbrone et al.4, there has been little success in the development of slow release agents for large molecular weight compounds. Furthermore, the polymers used in those studies, polyvinylpyrrolidone and polyacrylamide, are often inflammatory in animal tissues and usually permit only brief periods of sustained release. We now present a simple method for incorporating various proteins and other macromolecules into non-inflammatory polymers. Sustained release of biochemically active macromolecules has been demonstrated for periods exceeding 100 d.