The potential of human fetal bone cells for successful bone regeneration has been shown in vivo. In particular, it has been demonstrated that the seeding of these cells in porous poly-(L-lactic acid)/ ß-tricalcium phosphate scaffolds improved the bone formation compared to cell-free scaffolds in skulls of rats. However, even if the outcome is an improvement of bone formation, a thorough analysis concerning any immune responses, due to the implantation of a xenograft tissue, is not known. As the immune response and skeletal system relationship may either contribute to success or failure of an implant, we were interested in evaluating the presence of any immune cells and specific reactions of human fetal cells (also called human bone progenitor cells) once implanted in femoral condyles of rats. For this purpose (1) cell-free scaffolds, (2) human bone progenitor cells or (3) osteogenic human bone progenitor cells within scaffolds were implanted over 3, 7, 14 days and 12 weeks. The key finding is that human bone progenitor cells and osteogenic human bone progenitor cells do not trigger any particular specific immune reactions in immuno-competent rats, but are noted to delay some bone formation.