Experimental implants of polycarbonate covered with a thin metallic layer were inserted in the rabbit tibial metaphysis. The implants had either a magnetron sputtered 316 L stainless steel or commercially pure titanium surface (or an evaporated commercially pure titanium surface). The aim of the experiment was to investigate the interfacial arrangements between bone and the coatings used. Three months after implant insertion, the animals were sacrificed and the intact interface between bone and metal analysed using ultrastructural techniques. In the case of stainless steel a coat of 1-2 cellular layers separating the bone from the metal was found. Inflammatory cells were abundant as well as a wide proteoglycan coat lacking collagen filaments. In the case of titanium there were no cells in the interface which consisted instead of a proteoglycan layer of about 200-400 A width. Bundles of collagen appeared at a minimum distance of 1000-2000 A from the metal surface. Calcium deposits were sometimes seen in direct contact (resolution level 30-50 A) with the titanium oxide. There were no ultrastructural differences in tissue reactions when magnetron sputtered titanium surfaces were compared with evaporated ones.