Graft-vs-host disease (GVHD) and failure of donor stem cells to engraft permanently are two major obstacles to successful bone marrow transplantation. We evaluated the effect of a single large dose of anti-lymphocyte serum (ALS) on mice receiving various numbers of H-2 incompatible bone marrow cells. Most animals receiving lethal total body irradiation (TBI) and allogeneic marrow died within 45 days due to GVHD, A/J (H-2a), CBA/J (H-2k), and DBA/1J (H-2q) mice that were given ALS 6 to 24 hr before TBI and C57BL/6 (B6, H-2b) bone marrow 24 hr after irradiation survived in good health for more than 200 days. This result compared quite favorably with mice that received anti-Thy 1.2 and C-treated B6 bone marrow (90% survival at 100 days, 50% survival at 200 days). Engraftment of the allogeneic marrow was dose dependent. At 100 days after TBI, about 30% of the A/J mice given 2 x 10(6) bone marrow cells were complete chimeras, i.e., donor H-2 antigens could be detected on greater than 85% of the peripheral blood mononuclear leukocytes of these mice. However, at a dose of 1 x 10(7) B6 bone marrow cells, 95% of the A/J mice were complete chimeras. Spleen and bone marrow from B6 leads to A/J chimeras and B6 leads to CBA chimeras were unable to induce lethal GVHD in TBI-treated mice that were syngeneic to the recipient. However, these cell preparations caused lethal GVHD in third party mice indicating that the lack of alloreactivity was specific to the strain in which the unresponsiveness was originally induced.