Twenty-three patients have had calf or pig heterograft aortic valve replacements. Eight of these patients have required Starr-Edwards mitral ball-valve prostheses for associated mitral valve disease. There have been two hospital deaths and one late death. The 20 surviving patients have shown symptomatic improvement. Clinical evidence of incompetence is present in one patient. Pre-operative and post-operative ciné-aortograms show valve mobility with a wide central orifice. Heterografts have the advantages of homografts in being free from thromboembolism and from the need for anticoagulants; they exhibit some resistance to infection as compared with prosthetic valves. We think that heterografts, because of their wider range of size, can offer a better haemodynamic result with little clinical aortic incompetence. The long-term fate of aortic valve heterografts is not known with certainty, although the durability of homografts is better known. There is a possibility of late degeneration or rupture. If the valve continues to function normally, then it would be justifiable to recommend operation at an earlier stage in the progression of the patient's disease. At present, despite encouraging early results, we think the use of heterografts should be limited, like that of other prostheses, to those patients who show severe cardiac disability.