Actinidin was purified from kiwifruit (Actinidia deliciosa) by covalent chromatography, and its proteolytic effects on pork, chicken and beef myofibrillar proteins were examined at varying pH values. In the pH range of 3-4, actinidin thoroughly hydrolyzed all of the myofibrillar proteins, including myosin heavy chain and actin, in a non-specific manner. In the pH range of 5.5-8, actinidin selectively hydrolyzed myosin heavy chain into fragments of 72, 000-150, 000 molecular mass, whereas it showed little or no proteolytic effect on actin. In contrast, papain and bromelain non-selectively hydrolyzed myofibrillar proteins in the pH range of 2-8. These results suggest the possibility that actinidin, when used at an appropriate pH value, may tenderize meat without excess proteolysis of the myofibrillar proteins that results in loss of its texture and nutrients.