The regeneration of T cell subsets was studied with double immunofluorescence marker methods in 37 patients who received HLA matched T lymphocyte depleted bone marrow transplants (BMT) as part of the treatment for their haematological disease. A cocktail of anti-pan-T (CD6: MBG6) and anti-suppressor/cytotoxic-T cell (CD8: RFT8) monoclonal antibodies was used with rabbit serum as a source of cytolytic complement to achieve selective T cell lysis. The T8+ cells reached low normal values around 60 days post-transplant and remained within the normal range throughout the study (greater than 150 days). This observation is in contrast to our previously published results in patients who, after receiving BMT without efficient T cell depletion, had increased numbers of circulation T8+ cells from 60 days post-transplant. In the present study Leu-7+, RFT8- cells reached normal values rapidly but the reconstitution of T4+ lymphocytes was slow: low normal levels were reached only around day 150 following BMT. The degree of graft-versus-host disease (GVHD) seemed to be related to the number of residual T cells infused: two of the three patients who received the highest numbers of T cells developed Grade II III; otherwise GVHD was minimal. Among the clinical parameters studied cytomegalovirus (CMV) immune status moderately influenced reconstitution: at 55-90 days post-transplant T8+ cells were present at the upper normal levels in seven out of 15 patients receiving BMT from CMV seronegative donors, but in none of the 16 individuals receiving BMT from seropositive donors. CMV related complications were relatively uncommon. Thus the most significant factor in preventing 'T8+ cell overshoot' and T cell imbalance during regeneration appears to be the depletion of T (including T8+) lymphocytes from marrow. The differences of T8+ cell reconstitution in this and previous studies may reflect a different regeneration pattern from T cell precursors as opposed to inoculated mature T cells.