The development of methods for the production of monoclonal antibodies is having an important impact in the field of immunohaematology. Four separate areas are implicated. First, there is the use of monoclonal antibodies in blood transfusion, where antibodies within the ABO, Rh, Lewis, P, MN, Kell and Lutheran systems are available. Most of the monoclonal antibodies are of murine origin but the techniques for producing human monoclonal antibodies is now well established and this is especially valuable in the Rh system, with the production so far of anti-c, D, -E, -e and -G. Secondly, there is a great potential for the use of monoclonal anti-D to substitute for polyclonal anti-D in the prophylaxis of haemolytic disease of the newborn. The introduction of these antibodies will depend on clinical trials using both the IgG1 and IgG3 subclasses and on the ability to prepare antibody which is free of viruses and DNA. Thirdly, monoclonal antibodies are being used in basic research on red cell membranes to isolate and characterise blood group antigens. Finally, these antibodies are being used in bone marrow transplantation to purge the donor marrow of T-cells in order to reduce the incidence of graft-versus-host disease.