Five hydroxyapatite and two tricalcium phosphate ceramics were implanted in rat femora for one to six months. They were compared with each other in regard to biocompatibility and bone induction. After one month bone had grown directly on to the surface of all implants. After six months the on-growth of bone had increased with the exception of two dense HA materials. In these materials some bio-degradation had occurred with subsequent formation of a wide macrophage interlayer. Bio-degradation and biocompatibility of calcium phosphate ceramics seem to depend not only on density but also on other factors. Induction of bone could not be observed in soft tissue environment.