Antilymphocyte globulin is an immunoglobulin preparation prepared from heterologous serum after the animal (horse or rabbit) has been immunised with human lymphocytes, obtained from the thymus (antithymocyte globulin, ATG) or thoracic duct (antilymphocyte globulin, ALG). The rationale for the use of ALG in the treatment of chronic acquired marrow failure is based on its immunosuppressive activity and the fact that a proportion of cases of bone marrow failure, whether affecting single or multiple haemopoietic cell lines are due to immune-mediated suppression of haemopoiesis. In addition, in vitro studies have shown that ALG also has an immunostimulatory effect on lymphokine and haemopoietic growth factor production, and may therefore directly stimulate haemopoietic progenitor cells. ALG has been used for the treatment of aplastic anaemia and acquired chronic marrow failure affecting single cell lines namely pure red cell aplasia (PRCA), amegakaryocytic thrombocytopenia and chronic neutropenia due to immune inhibition of granulopoiesis ('acquired white cell aplasia'). ALG is used for treatment of non-severe aplastic anaemia (NSAA) and in those cases of severe aplastic anaemia (SAA) where allogeneic transplantation is not possible or is not indicated. Treatment with ALG results in 75% long term survival for NSAA and 40-50% for SAA although there is a very severe subgroup of SAA defined by peripheral blood neutrophils of less than 0.2 x 10(9)/l who rarely benefit from ALG therapy. For those patients who do not respond a second course of ALG can be given later using ALG from a different animal source.(ABSTRACT TRUNCATED AT 250 WORDS)