Microstructures of the early external callus after diaphyseal fractures of human long bone were investigated by using scanning electron microscopy, X-ray diffraction, and transmission electron microscopy, It was found that the main structural framework of the human early callus consists of disordered, mineralized collagen fibrils with a small fraction of regions of ordered collagen fibrils, X-ray diffraction analyses show that hydroxyapatite containing some carbonate impurity has been the dominant crystalline phase in the human early callus. In addition, a small amount of brushite phase was detected, Selected area diffraction analyses indicated that hydroxyapatite microcrystals were embedded in microfibrils with a diameter of 4.5 nm and well-banded fibrils, whereas brushite particles of 15-20 nm in an irregular shape were located in the noncollagenous organic matter around the nonmineralized, ordered collagen fibrils, The spatial distribution of the brushite particles in the human early callus was for the first time determined. The brushite particles probably serve as the reservoir of calcium and phosphate ions for subsequent mineralized periods rather than the precursor of hydroxyapatite. (C) 1995 Academic Press, Inc.