Peripheral blood T-lymphocyte subsets, evaluated by means of a series of monoclonal antibodies, were assessed in 14 patients affected by thalassaemia intermedia, 7 of them previously splenectomized. A significant reduction of T⁺₄ cells (‘helper’ T cells) was found in almost all patients, whereas T⁺₈ cells (‘suppressor/cytotoxic’) showed a marked increase only in splenectomized subjects. Together with these quantitative T-subset abnormalities, which seemed to be partly affected by either splenectomy or high serum iron levels, an unusual circulating T-cell subpopulation labelled by T₆ monoclonal antibody was detected in all patients. Complete disappearance of T- cells (‘thymo-cyte-like’ T lymphocytes) and normalization of the T₄/T₈ ratio was observed after ‘in vitro’ incubation of patient’s lymphocytes with a crude thymus extract (Thymostimulin). This would suggest the presence of a so far unreported thymus-dependent defect of T-lymphocyte phenotypic maturation occurring in thalassaemia intermedia.