Electron microscopic were made of the occurrence of reversible and irreversible injury in human aortic valves subjected to cryopreservation after various periods of warm ischemia (i.e., the time elapse between death and the beginning of cryopreservation) in preparation for use as allografts. The data obtained showed that there was virtually no morphologic injury in valves with harvest-related warm ischemic times less than 2 hours and minimal irreversible cellular injury in valves exposed to 12 hours or less of warm ischemia. Irreversible injury increased markedly after 12 hours of warm ischemia. If cellular viability is critical to homograft durability then harvest-related warm ischemia may need to be restricted to less than 12 hours.