Abstract: Biological materials are treated with an organic chemical agent, dimethyl sulfoxide, to delay the onset of in vivo calcification. The biological materials so treated become resistant to calcification over a prolonged period with no known deleterious effects to the host. The procedure involves the incubation of the natural biological materials in the chemical agent, prior to or after glutaraldehyde fixation, at room temperature or at temperatures up to 60.degree. C. Materials that can be treated with this process include biological materials intended for bioprostheses use, such as porcine aortic valves, porcine aortic valve leaflets, bovine, equine and porcine pericardium, aortic homografts, reconstituted collagen, and the like suitable for invasive, or in-dwelling use in a human or animal body.