Complement-mediated cytolysis has been used to remove T-lymphocytes from suspensions of human peripheral blood and bone marrow. Selective T-cell removal was investigated by three monoclonal antibodies. OKT3, MBG6 and OKT11A. All three removed greater than 90% of T-cells but combinations were necessary to kill greater than 99% of T-cells in vitro. The macrophage-granulocyte and erythroid colony forming cells of the bone marrow were spared. The method can be applied on bulk BM samples during clinical BM transplantation and will be useful to establish whether the virtually complete removal of T-lymphocytes totally prevents transplant associated graft-versus-host disease in man.