T-cell subpopulations were studied in six patients after allogeneic bone marrow transplantation for aplastic anemia and acute leukaemia, by means of receptors for IgG (TG-cells) and IgM (TM-cells). Significant modifications of TM-TG levels were found: a 2 to 10 fold increase of TG cells and a 2 to 10 fold decrease of TM cells from pre-transplant levels could be observed. The TM-TG imbalance appeared together with the clinical signs of GVHD: the level of TM-depression (but not the level of TG increase) correlated with the severity of GVHD. The TM-TG modifications were observed as long as 3 years after transplantation, also in the absence of signs of GVHD. In addition, all six patients were found to have severely impaired pokeweed-induced B-cell differentiation. The clinical implications and the possible lack of T-helper activity are discussed.