Aortic Allograft (Homograft) : In the earlier years fresh antibiotic preserved aortic allografts were tried. As the shelf life is limited, availability of all sizes at all times could not be guaranteed. At present cryopseservation is preferred. The valve is harvested under sterile conditions and is frozen using liquid nitrogen tissue culture medium containing 10 per cent dimethyl sulploxide (DMSO). It is stored a1 -180°C. They are durable and free from structural deterioration in 80 per cent of the patients at 15 years.