The relative distribution of T cell subsets, as defined by the monoclonal antibodies OKT, was analyzed in peripheral blood lymphocytes of 8 children after bone marrow transplantation for aplastic anemia. The percentage of peripheral blood lymphocytes binding OKT3 (directed against a common T cell surface antigen and defining most peripheral T cells) reached normal values shortly after transplantation. In 5 patients, lymphocytes binding OKT8 (directed against an antigen present on the suppressor/cytotoxic T cell subset) were found in high proportion, and lymphocytes binding OKT4 (detecting an antigen present on inducer/helper T cells) in low percentage. This resulted in an inverted ratio OKT4/OKT8 compared with that of lymphocytes from normal individuals. In all patients the lymphocytes bound abnormally high amounts of OKT10 (directed against an antigen present on all thymocytes but not on mature peripheral T cells). In the course of time, a trend towards normalization was observed for all parameters investigated; however, the kinetics of the recovery showed a marked heterogeneity. From the analysis of this phenomenon, it is likely that, among other conditions yet unknown, a minimum of 20% of OKT4-positive lymphocytes is required for a normal proliferative response to T cell mitogens in vitro. No other correlation was found between any lymphocyte phenotype, as defined by the OKT antibodies, and proliferative response in vitro. Furthermore, lymphocytes from the patient with chronic graft-vs-host disease (greater than 50% OKT8 positive) failed to suppress the proliferative response to mitogens and antigens of the lymphocytes of the histo-identical bone marrow donor.