Six (3 control and 3 explanted) bioprostheses were examined histologically after embedding the complete valve in "plastic" and sectioning across the valve at 90 degree to the horizontal plane, and then horizontally across a stent post, with a low speed saw. The prostheses, porcine or pericardial, had been in place 52 to 121 months. In addition to cusp changes of degeneration and calcification that have been previously reported, the authors found extensive pannus deposition on the cloth covering the inflow surface of the prostheses, with some extension onto the biologic components; evidence of insudation of tissue fluid and ingrowth of fibroblasts; deposition of collagen; and infiltration of mononuclear cells in the interstices and interfaces. Non-biologic components showed changes which may have affected prosthesis function. The synthetic material covering the metal/plastic frame showed interstitial mononuclear cells. Two of the three explanted valves showed changes in the polymeric plastic, and one in the metal components, including accentuated notching along the surfaces when compared to unimplanted prostheses. A larger number of prostheses must be examined before definite conclusions can be drawn.