The functional activity of B and T lymphocytes from the blood of eight patients, who had successfully been treated with allogeneic bone marrow for severe aplastic anaemia or acute leukaemia, was studied in pokeweed mitogen (PWM) driven polyclonal immunoglobulin synthesis. Activity of B cells was measured as IgM and IgG synthesis by a standard number (40 X 10(3] of patient lymphocytes in the presence or absence of healthy donor T cells. In addition, the frequency of PWM reactive B cells, giving rise to IgM and/or IgG producing daughter cells, was estimated by limiting dilution analysis. With this method, it was found that only a small percentage (1-3%) of peripheral blood B cells from healthy individuals is reactive to PWM. In the patients, both parameters for B cell reactivity were decreased during the first 40 weeks after bone marrow transplantation. As parameters for T cell activity, help and suppression on the Ig production by healthy donor lymphocytes were tested. In most patients, T helper cell activity was strongly decreased, whereas some patients had excessive T suppressor cell activity. The observed functional activities were only partially correlated with the marker profile of the T cell populations, as detected by reactivity with monoclonal antibodies. Each patient had a distinct, individual pattern of reconstitution of these functions. There was no positive correlation between Ig production in vitro and the capacity to form antibodies in vivo, nor between the other in vitro findings and clinical features, such as the occurrence of infections or graft versus host disease.