This study deals with the histological reaction of subcutaneous connective tissue, muscle, peritoneum, nervous tissue, and bone to implants of a collagen derivative obtained from bovine bone, which has been called Collatissue A. The tissue response was in two stages, first, a fibroblastic proliferation about the implant and second, an infiltration by small and large mononuclear cells. Capillary proliferation occurred around the implant. The material gradually lost its normal histological structure, became amorphous, and was replaced by fibrous tissue. There was no polymorphonuclear cell response and giant cells appeared rarely. Silk, catgut, and tantalum foil were used as control materials. They induced a typical foreign body response, which included polymorphonuclear and giant cells. The antigenic properties of Collatissue A were studied to a limited extent with negative results. Collatissue A has certain physical properties, such as tensile strength and flexibility, that suggest possible surgical applications of this material.