Peripheral T-lymphocyte reconstitution after bone-marrow transplantation, 12 allogeneic and 13 autologous, was studied by indirect immunofluorescence assay using mouse monoclonal antibodies. Abnormal counts were detected in the two major sub-populations of T-cells i.e. helper and T cytotoxic-suppressor lymphocytes, defined by monoclonal antibodies (alpha Leu 3a and B 9.2), and in DR antigen-positive T-cells. The pattern of T-lymphocytes replenishment was identical for both types of transplant, and was not affected by Graft Versus Host disease (GVHD).