The third ossification mode, known as transchondroid bone formation, is displayed chiefly in the bone morphogenetic protein (BMP)-induced heterotopic bone formation model. This paper describes the results of histopathological, histochemical, immunohistochemical, and in situ hybridization examinations of BMP-induced heterotopic bone in mice. The research focuses on the localization of typical matrix proteins (peptide and its mRNA) of cartilage and bone--type-I and type-II collagen, osteocalcin and osteopontin--in the chondroid bone matrix.