The T-cell-accessory-cell interaction in mixed lymphocyte cultures was investigated in 25 patients following autologous bone marrow transplantation (ABMT) using autologous bone marrow treated in vitro with the cyclophosphamide derivative ASTA Z 7557. In a previous study using the same group of patients, T cells failed to synthesize interleukin-2 (IL-2) and proliferate in response to CD3- and CD2-mediated stimuli even in the presence of exogenous IL-2. To investigate whether this defect in IL-2 synthesis and proliferation was caused by defective cell-to-cell interactions, we analyzed mixed lymphocyte reactions (MLR) using T cells and irradiated non-T cells. When normal T cells from 10 different healthy subjects were challenged with allogeneic normal non-T cells, IL-2 production and proliferation were observed. In contrast, when normal T cells were cultured with non-T cells derived from patients found between 20 and 330 days after ABMT, no IL-2 secretion and no proliferative responses could be seen. The addition of lymphokines such as interleukin-1 (IL-1), interleukin-3 (IL-3), tumor necrosis factor (TNF), granulocyte-macrophage colony stimulating factor (GM-CSF), and interferon-gamma (IFN-y) did not improve the reactions. Furthermore, when patients' T cells were incubated with normal, irradiated non-T cells, defective IL-2 synthesis or proliferative response was obtained. However, when IL-2 was added to these cultures, an improvement in proliferative reactions was observed. Taken together, these new data provide additional evidence that T cells early in ontogeny possessed an intrinsic defect in IL-2 synthesis and that physical cell-to-cell contact between patients' T cells and allogeneic accessory cells induced functional responsiveness to exogeneous IL-2.