Compact cylindrical implants (5 x 15.5 mm) of seven calcium phosphate ceramics of different formulations that had been implanted for 6 weeks without fixation in the tibia of dogs were examined histologically. The tissue compatibility of the implant turned out to be dependent on the mineralogical and chemical composition of the material: the ceramic material was biocompatible at a CaO/P2O5 ratio between 2:1 and 4:1, the optimum ratio being about 3:1 (tricalcium phosphate). In addition, cylindrical tetracalcium phosphate implatns with an entirely porous structure were implanted in the same manner. After 6 months, they were resorbed to a minor extent. Implants of different calcium phosphate mixtures, on the other hand, were resorbed to a large extend after the same time. The ceramic material was replaced by mineralised bone tissue which had formed directly on the ceramic implant as well as within the pores. There were no foreign body reactions. Investigations with segment-shaped implants which were implanted in the tibia of dogs and fixed with AO-plates or splints extra-cutaneously for about 8-10 weeks, provided information abouth the maximum stress that can be borne by the implants. While implants with a porosity of 75 percent did not sustain the stress after the fixation had been removed, those with a porosity of 45 percent could be subjected to physiological stress after removal of the splints.