In 15 children with thalassemia major (age 4–17 years) a detailed analysis of different immune functions was performed: phagocyte function, specific cellular immunity, humoral defense system. All patients had been subjected to a desferrioxamine therapy and a high transfusion regimen. Examination of neutrophil function included adherence, random migration, chemotaxis, killing of Escherichi coli and production of superoxide radical; these neutrophil functions were shown to be normal. In addition, lymphocyte proliferation in response to different lectins (phytohemagglutinin, concanavalin A, pokeweed mitogen) was identical in patients and controls. However, the number of circulating T-lymphocytes, helper T-cells and B-lymphocytes was increased in some patients. This phenomenon probably reflects an unspecific stimulation of the antibody-producing cells by repeated blood transfusions.