SUMMARYAntilymphocyte sera (ALS) were prepared by different methods and examined for their myelotoxicity by bone marrow colony-forming assay, both before and after absorption with B cell antigens. The results show that myelotoxicity can readily be removed from some, but not all, ALS. The source of antigen and the immunisation protocol used determine the degree of myelotoxicity and hence the number of absorptions required. Non-myelotoxic ALS may be suitable for the prevention of graft-versus-host disease (GVHD) by in vitro treatment of bone marrow before transplantation.