MARCH 1E11 is an IgM monoclonal anti-human lymphocyte antibody of rat origin with the capacity to utilise both human and rabbit complement. The antibody reacts with all thymocytes and with all peripheral blood T and B lymphocytes. The treatment of human bone marrow or human peripheral blood mononuclear cells (PBMC) with MARCH 1E11 and either pooled human serum or autologous serum as a complement source resulted in cytolysis of greater than 99% of OKT3-positive lymphocytes. Under these conditions, progenitor cell recovery (colony-forming unit (CFU-c), burst forming unit--erythroid (BFU-e) and colony-forming unit--mixed (CFU-mix)) was greater than 90% of that of untreated cells. The response of treated marrow or PBMC to phytohaemagglutinin stimulation and in mixed leucocyte reactions demonstrated a reduction in thymidine incorporation to values similar to those obtained for unstimulated cells. The antibody does not cause modulation of the cell surface antigen and does not react significantly with non-lymphoid tissues. This monoclonal antibody may be useful for in vitro elimination of T lymphocytes from allogeneic bone marrow used for transplantation. The antibody may also be useful for treatment protocols requiring lymphoid depletion or immunosuppression as in organ transplantation.