Summary. The passive administration of anti‐D immunoglobulin is effective in preventing Rh alloimmunization and therefore preventing haemolytic disease of the newborn. However, diminishing availability and increased demand render an alternative source of anti‐D desirable. We report the production of two human monoclonal IgGl antibodies specific for Rhesus antigen D. Data are presented on the serological reactivity of these antibodies and on their ability to promote in vitro biological activity. Antibody derived from both lymphoblastoid cell lines and the hetero‐hybrid cell lines were compared for both ESD‐1 and ESD‐4. The ability of the antibodies to detect rhesus related antigens in animal tissue was also assessed. We present some evidence for altered reactivity of the ESD‐4 antibody following fusion of the lymphoblastoid parent line to a murine plasmacytoma.