Antilymphocyte, antigranulocyte and antiplatelet alloantibodies, T lymphocyte subsets, expression of HLA-DR antigens on T lymphocytes and NK cell function were determined in 11 homozygous β-thalassemic children multitransfused ab initio with Erypur-filtered leukocyte-free red cell units (group A) and in 13 similar children multitransfused with standard packed red cell units (group B). No antibodies were found in group A patients, whereas 69% of group B patients were immunized. The two groups did not differ significantly with regard to the other test results. Considered together, thalassemia patients showed a percentage of T4+ cells and a NK cell function that were significantly lower than those found in a reference group of 16 healthy male blood donors. Thalassemics moreover showed a higher than normal percentage of T3+, T4+ and T8+ cells expressing HLA-DR antigens. The results indicate that leukocyte-free red cells should be the treatment of choice for prospective recipients of multiple transfusions, since they are capable of preventing (or delaying) the production of alloantibodies against leukocytes and platelets. From the data of the present study, it does not seem that the transfusion of leukocyte-free red cells is capable of preventing the abnormalities of some immunological tests that occur in some multitransfused patients. Further investigations, however, are needed to draw conclusions on this problem.