To develop an artificial bone substitute that would be gradually degraded and replaced by the regenerating natural bone, a carbonate apatite type I atelocollagen (82/12 in v/v%) composite was designed. The carbonate apatite synthesized at 58 degrees C demonstrated a crystallinity similar to that of natural bone. Type I atelocollagen was purified from human umbilical cords. Carbonate apatite-atelocollagen composite rods (diameter 6 mm x height 10 mm) were irradiated by ultraviolet ray (wave length 254 nm) for 4 hr to increase the collagen fibrillar cross links. Rabbit tibiae were dissected to prepare an artificial total bone defect (length 10 mm). The composites and porous hydroxyapatite rods, sintered at 1200 degrees C, were implanted into the defects and the tibiae were fixed by external osseofixators. The implanted composites were gradually degraded in the lesions, and the regenerated bone totally replaced the defects within 6 weeks, while the hydroxyapatite rod implanted lesions were not replaced by bone. No specific histologic abnormalities appeared in either the hydroxyapatite or composite rod implanted lesions.