Peripheral blood T lymphocyte subpopulations were analysed using OKT3, OKT4 and OKT8 monoclonal antibodies in eleven patients undergoing autologous bone marrow transplantation following high dose chemotherapy +/- total body irradiation for bronchial carcinoma or as intensification therapy for acute myeloblastic leukaemia. Marked inversion of the OKT4:OKT8 ratio was noted in all patients following the procedure. This is due to both a fall in the number of OKT4 positive cells and a rise in the number of OKT8 positive cells in all but one patient. Sequential analysis of six patients shows a return to the normal ratio in two with time and a trend towards normalization in the others. We suggest that the subpopulation inversion noted following bone marrow transplantation is an effect of marrow regeneration and is not causally related to graft versus host disease.