Growth cartilage (GC) cells of young rabbits were cultured in vitro and their homogenates were injected into mice. Hybridomas were prepared by the cell fusion technique between the myeloma cells and the spleen cells of the immunized mice. Monoclonal antibodies (MoAbs) were produced by the hybridomas in the peritoneal cavities of the mice, and some of these, temporarily named MoAbs A, B, D, N, P, and S, were studied. The localization of the antigens of each of the MoAbs in the GC or adjacent resting cartilage (RC) was examined by indirect fluorescent antibody staining. The molecular weight of the antigens was examined by immunoblot staining after SDS-polyacrylamide gel electrophoresis. MoAb A and MoAb N stained RC cells and GC cells, except calcified GC. MoAb B stained the hypertrophic and calcified GC, and matrices in the RC and proliferating GC. MoAb D stained the calcified GC. MoAb P and MoAb S stained the RC cells and the matrices in the GC, intensively in the hypertrophic GC and perichondrium. The molecular weights of the antigens of MoAbs A, P, and S were 40-70 KD, 35-40 KD and 30 KD, respectively.