To elucidate the process of endochondral ossification in human osteophytes we have studied the expression of parathyroid hormone-related protein (PTHrP), its receptor (PTHr), and fibroblast growth factor receptor 3 (FGFR3). Osteophytes from patients undergoing total knee replacement (n=13), and fetal growth plate cartilages (n=4) were processed for safranin O staining and immunohistochemistry. Chondrocytes and their matrix were preferentially stained for PTHrP in the middle and deep zones of the osteophytes examined. Ossified areas did not show a positive staining. In fetal joints the cartilaginous surface and the perichondrium as well as the osteoblasts in the trabecular bone were positive. PTHr was expressed at large in chondrocytes and osteoblasts of all osteophytes and fetal joints. Cells of the perichondrium were also positive. The FGFR3 antibody stained only single chondrocytes in some osteophytes, and groups of cells in others. In fetal samples, chondrocytes of the proliferating and the hypertrophic zone showed staining for FGFR3. This is the first report on the expression of PTHrP, PTHr, and FGFR3 in human osteophytes. As in fetal joints these mediators might regulate proliferation and differentiation of chondrocytes playing an important role in osteo(chondro)phyte growth.