Disclosed are methods by which compounds/molecules capable of binding antigens, for example antibody type compounds/molecules, can be purified, extracted and/or selected. The methods may be used to purify, extract or select a specific type (or types) of binding agent from a mixed composition. The methods may be used to extract or purify specific binding agents from mixed compositions, which compositions comprise other agents capable of binding other antigens. The methods may find particular application as methods for the purification of blood group antigen antibodies.