The biological valve "BAKS" has the following specific features: (1) it is practically devoid of immunogenicity due to fermentative-chemical treatment; (2) it is completely treated with glutaraldehyde through gradual soaking in solutions of increasing concentration; (3) the biological part is fashioned taking into account the biomechanics of the aortic root; (4) the polypropylene stent is functional due to the use of the principles of bionics in valve construction; and (5) practically no thromboemboli occur, because of the special morphologic organization of the cuff and the distant location of the bioprosthesis leaflets from the stent edge. In the Department of Heart Surgery of the National Research Center of Surgery, "BAKS" valves have been used in 237 patients undergoing isolated mitral valve replacement for rheumatic heart disease. Of these 35% were younger than 35 years, 93.7% belonged to New York Heart Association functional class IV, and 24% had had a previous heart operation. The 10-year survival rate without hospital mortality was 68.94% +/- 4.86%. Total follow-up was 11,232 patient-months (range; 6 to 118 months; mean, 57 months; 97% complete). Main causes of death and reoperation were (per patient-year) infectious endocarditis, 3.2%; spontaneous bioprosthesis degeneration, 0.96%; mechanical dysfunction resulting from damage in the manufacturing of the prosthesis, 0.74%; left atrial thrombosis and/or thromboemboli, 0.5%; sudden death, 0.2%; cardiac insufficiency, 0.3%; and other, 0.4%.