This invention comprises a method of using tubular material (200) to replace a heart valve (118, 120, 162) during cardiac surgery. To create a replacement atrioventricular (mitral or tricuspid) valve (120, 160), the tube inlet is sutured to a valve annulus (121, 162) from which the native leaflets have been removed, and the tube outlet is sutured to papillary muscles (126, 128, 170) in the ventricle (112, 152). To create a semilunar (aortic or pulmonary) valve (118), the tube inlet is sutured to an annulus from which the native cusps have been removed, and the tube is either 'tacked' at three points distally inside the artery, or sutured longitudinally along three lines; this allows the flaps of tissue between the three fixation points at the valve outlet to function as movable cusps. These approaches generate flow patterns that closely duplicate the flow patterns of native valves. A preferred tubular material (200) comprises submucosal tissue from the small intestine; this can be taken from the same patient who is undergoing the cardiac operation, to reduce antigenicity. Animal or cadaver intestinal tissue can also be used if properly treated, or a biocompatible synthetic tubular material can be used. This invention also discloses a properly treated intestinal tissue segment or synthetic tubular segment (500) suitable for implantation as a replacement heart valve, enclosed in a sealed sterile package (510).