Fifty-four patients allografted for leukaemia were evaluated at various intervals after bone marrow transplantation for the presence of host haemopoiesis using red blood cell and cytogenetic markers. Out of 40 patients in remission, 10 showed functional host and donor haemopoiesis (mixed chimerism), whereas in the other 30 (complete chimerism) host haemopoiesis was never detected. Seven of the 14 evaluable patients who relapsed showed the reappearance of host haemopoiesis at the time of relapse. Analysis of the dose of total body irradiation (TBI) indicated that patients who achieved mixed chimerism, whether or not they relapsed, had received significantly lower doses than those with complete chimerism. However, some patients with complete chimerism had received a TBI dose equivalent to the dose received by those with mixed chimerism, suggesting that the TBI dose is not the only factor determining the reappearance of host haemopoiesis. The data on chimerism and relapse suggest that there is heterogeneity in radiosensitivity between normal marrow cells and leukaemic cells, and also within the different types of leukaemia. The incidence/severity of acute and chronic graft-versus-host-disease (GVHD) was significantly higher in patients with complete chimerism than in mixed chimeras, suggesting that mixed chimerism may play a role in the development of tolerance. Alternatively the absence of GVHD (i.e. tolerance) may be responsible for the persistence of host haemopoietic cells.