Different groups of C57BL/Ka mice received daily fractions of 2 Gy total lymphoid irradiation (TLI) in a total dose of 34, 24, or 14 Gy. On the day after the last irradiation, 30 X 10(6) allogeneic (BALB/c) nucleated bone marrow cells were infused into the irradiated animals. When the last one or two fractions of the radiation schedule were given to the whole body (combined total lymphoid-total-body irradiation, TLBI): (1) stable bone marrow chimerism with a higher number of donor-type cells in the peripheral blood was induced in a higher percentage of mice that had received 34 Gy TLBI compared with mice that received 34 Gy TLI. (2) bone marrow chimerism could also be induced after 24 Gy or 14 Gy TLBI, whereas 24 and 14 Gy TLI alone were ineffective. The tolerance to the TLBI schedules was excellent and no clinical signs of graft-versus-host disease were noticed. It is concluded that the addition of TBI can facilitate bone marrow engraftment after TLI and drastically reduce the number of radiation fractions needed to obtain successful chimerism after allogeneic bone marrow transplantation in mice.