Hydroxyapatite (Ca₅(PO4)₃(OH)₂ is a form of calcium phosphate that can be used for the separation or purification of proteins, enzymes, nucleic acids, viruses, and other macromolecules

Bio-Gel crystalline hydroxyapatite is chemically and thermally stable, is compatible with a wide range of aqueous and organic solvents, and can be sanitized in NaOH. It has a pH tolerance of >5.5 and is autoclavable. Three types of media are available:

Bio-Gel HT gel— Ideal for the purification of proteins, nucleic acids, and other macromolecules, Bio-Gel HT is prepared by the method of Tiselius (1956). It is suspended in 10 mM sodium phosphate buffer containing 0.02% NaN3 and should be stored at 4°C.

Bio-Gel HTP gel— This is the material described by Tiselius, which has been dried by a unique process developed by Bio-Rad. It may be stored without refrigeration. When suspended in buffer, it has characteristics similar to Bio-Gel HT gel; 1 g hydrates to 2–3 mL.

DNA grade Bio-Gel HTP gel— A dry powder with a smaller particle size than HTP gel, this gel significantly increases its capacity and enhances its selectivity for double-stranded DNA. Due to its small particle size, it is recommended for batch mode chromatography or for use with very short columns.

Hydroxyapatite chromatography can be used at any stage, from initial capture to final polishing. Hydroxyapatite has unique selectivity and resolution, and often separates proteins that appear to be homogeneous by other chromatographic and electrophoretic techniques.