The present study aimed to describe the ultrastructural localization of alkaline phosphatase (AP) activity in articular-epiphyseal growth cartilage of the commercial pig and the minipig of wild hog ancestry, comparing areas with a normal endochondral ossification with those where the calcification of the matrix is insufficient, as in osteochondrotic cartilage. Intense AP activity was primarily present in the cytoplasm, the plasmalemmae the long cellular processes and the matrix vesicles budding off from proliferative and hypertrophic chondrocytes in those areas of cartilage where normal calcification appeared. In the osteochondrotic cartilage, the only detectable AP activity was restricted to a few morphologically viable hypertrophic cells in the surroundings of the lesion. The lack of AP activity could partially explain the insufficient calcification of the osteochondrotic cartilage.