Immunological reconstitution was studied with regard to T cell subsets and their functions in patients who received intensive therapy and autologous or allogeneic marrow transplantation. One of the distinct features was the imbalance of T cell subsets. Long-standing inversion of the OKT4/OKT8 ratio was characteristic in both autotransplant and allotransplant patients. Significant differences were observed in recovery of T cell subsets and mitogenic responses between autotransplant and allotransplant patients and between transplant patients and normal controls. In contrast, low reactivities of mixed lymphocyte culture (MLC) recovered to normal levels within 1 yr in both groups of patients. Then the role of suppressor cells was investigated. During early posttransplant periods, MLC suppressor cells were operative in association with the development of low MLC reactivities because suppressor activity was inversely correlated with MLC reactivities at a significant level. Characterization of these MLC suppressor cells revealed that they were an OKT8- and Ia-positive, radioresistant T cell subset of peripheral blood lymphocytes from autologous and allogeneic marrow recipients. These observations suggest that the imbalance of T cell subsets and their abnormal reactivities may by responsible for the development of immunodeficiency and immunodysregulation in bone marrow transplant patients.