The St. Jude Medical cardiac valve prosthesis is a low profile, lightweight, bi-leaflet, central-flow device constructed entirely of pyrolytic carbon. In vivo testing was performed in 24 cows in which the tricuspid valve (n = 14) or mitral valve (n = 10) was replaced by a St. Jude Medical prosthesis with a 26 mm tissue annulus diameter. Eight animals died in the operative period. The 16 surviving animals were sacrificed at 4, 8, 12, or 18 weeks. There were no valve-related deaths nor mechanical valve failures. Anticoagulation was not used in the postoperative period during which time monthly laboratory parameters were obtained including hemoglobin, hematocrit, white cell count, red cell count and indices, lactic acid dehydrogenase, serum haptoglobin, and, additionally at sacrifice, platelet and red cell morphology. Values indicated this prosthesis did not cause hemolysis. At sacrifice the pulmonary arterial tree and peripheral organs showed no evidence of thromboemboli. There was smooth endocardial ingrowth on all valve sewing rings. One valve had thrombus formation on the valve sewing ring. Nine animals were anesthetized prior to sacrifice and underwent open cardiac catheterization. Ventriculography (n = 6) showed minimal transvalvar reflux, and atriograms (n = 6) demonstrated central flow. Transvalvar gradients were 0 to 22 mm Hg (n = 9). One valve of an 18-week tricuspid implant was examined for wear; valve life was calculated to be greater than 2500 years. These data in conjunction with in vitro test results justify clinical trial of the St. Jude Medical prosthesis.