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Of the 14 patients with aplastic anaemia treated in our hospital with anti-thymocyte globulin (ATG), four were refractory to random platelets before therapy due to the presence of leucocyte antibodies. In contrast to the ten nonrefractory patients in whom no success was obtained, three of the four refractory patients showed haematological improvement after ATG. Additionally, two patients could be substituted again with random platelets. The other two hardly needed platelet-transfusions after ATG, and they were given HLA-compatible platelets. To determine the degree of immunosuppression, these four patients were tested for the presence of antibodies against leucocytes and two endemic viruses, i.e., mumps and rubella virus. Before ATG, all sera reacted with almost the whole leucocyte testpanel. After treatment, the leucocyte antibodies disappeared completely in three patients. In one patient there was no dramatic change. In all patients, however, the antibody-titre against the mumps and rubella viruses remained constant and there was only a slight decrease in total IgG content in the three "suppressed" patients. We conclude that it might be worthwhile to study systematically the selective immuno-suppressive effect of ATG.