The mitral valve was replaced by a pig aortic valve in 33 patients at Groote Schuur Hospital. Eleven of the failed heterograft aortic valves were examined at intervals of from 2 to 32 months after insertion. Fourteen control pig aortic valves were also examined. Electron microscopy was performed on two of the failed heterograft valves and three control pig valves. Failure of the heterograft was due to stretching and deformation of the cusps with resultant valvular incompetence. Stretching of the cusp was a result of reduction in the amount of its collagen content. The elastic tissue appeared little altered. A microscopic layer of fibrin thrombus was present on the surface of 8 of the 11 valves. Only 2 of the 11 valves showed invasion of the graft by immunologically competent cells. No valve showed any sign of infection or calcification. The denatured collagen of the heterograft has a low antigenicity and also, infortunately, a limited durability.