The EBV immortalization technique was used to produce stable clones, from B lymphocytes, secreting human monoclonal antibodies to Rh(D), Rh(G), Rh(c), Rh(E), Kell, A and A1 blood group antigens. These clones were obtained from peripheral blood lymphocytes of hyperimmunized plasmapheresis donors or from spleen lymphocytes of immunized patients. Mean levels of antibody concentration varied between 4 and 50 micrograms/ml. The antibodies obtained were of IgG1, IgG2, IgM or IgA class. Most of the clones have been stable for growth and antibody production during long periods of continuous culture, extending upto 4 years. Hybridization of two clones was effected with the human lymphoblastoid cell line KR-4 and with the mouse myeloma X63-Ag8.653, but did not result in any marked improvement of clone characteristics. One of the anti-Rh(D)-producing EBV-transformed clones was used to produce an anti-Rh(D) typing reagent which has proved satisfactory for 2 years in routine blood typing in several laboratories.