The lifetime of bioprosthetic heart valves is limited by primary tissue failure and calcification of the valve leaflets. There are indications that synthetic elastomeric materials may also be subject to this problem. The mechanism of calcification is not known, but it is of interest that calcification can be induced in tissue even in the absence of cellular mechanisms, outside the body. Many hypotheses relate to inhibitory or promotory factors rather than primary instigators of calcification and none has led to a satisfactory solution of the problem. The study of calcification in replacement valves generally utilises in vivo test methods i.e. complex biologic systems. This creates difficulty in defining the primary factors involved. The use of in vitro test methods, including a novel fatigue tester method, has been reviewed. Various test media have been used, including simple salt solutions (allowing definition and controlled modification of the calcification medium) and bovine plasma. Comparison of static and dynamic in vitro methods with the rat subcutaneous implant model indicated a lower degree of calcification in vitro: the calcification achieved was, however, significantly greater than similar material not subject to calcification processes. Dynamic in vitro tests produced greater calcification than static in vitro tests. Porcine aortic valve material, in static tests, behaved similarly to bovine pericardium. In vitro calcification testing has a useful role to play in the economic screening of new materials or modifications of existing materials prior to in vivo testing. It may also aid the definition of the mechanism of calcification and hence the development of solutions to the problem.