The first component of complement C1s has been shown to degrade type I and type II collagens (Yamaguchi et al. 1990), the latter of which is a major constituent of the cartilage matrix. In order to understand the physiological roles of C1s in cartilage resorption, the expression of C1s was examined by immunohistochemistry in the primary ossification center where the matrix is removed and replaced by bone marrow. Hypertrophic chondrocytes, endothelium and hematogenous elements in the capillary buds were intensely stained by a monoclonal antibody against C1s. Matrix metalloproteinase 9 (MMP-9, 92kDa gelatinase/type IV collagenase) was also immunolocalized in hypertrophic chondrocytes, mesenchymal cells in the primitive bone marrow and the cartilage matrix adjacent to the marrow. In addition, C1s was found to activate the zymogen of MMP-9. These observations suggest that C1s and MMP-9 coordinately participate in matrix degradation in cartilage.