Histones were detected in the HCl extract of bone matrix. The mecharusm of their incorporation into the matrix was investigated by in vitro binding experiments. We purified from the HCl extract ofbovine bone a group of basic proteins, which were identified as histones by Western blotting using antilustone antibody. Histones were incubated with synthetic hydroxyapatite, and were found to be bound to the crystals. Bone non-collagenous protems were separated by gel electrophoresis and transferred to a membrane. The membrane was incubated with histones, and the histones bound were detected with antihistone antibody. Histones were specifically bound to proteoglycans. Histones derived from dead bone cells may be incorporated into bone matrix by their affiruty to hydroxyapatite and/or to proteoglycans.