Abstract: A single lumen implantable and biocompatible breast prosthesis composed of an outer membrane of silastic, medical grade silicone, and an inner filling material selected from the group consisting of purified reconstituted collagen gel and a purified gel of poly-alpha amino acid homopolymers or random copolymers having a molecular weight of from 5,000 to 400,000. The purified reconstituted collagen gel has a collagen concentration of from 20 to 100 milligrams per milliliter of gel.