To study the mechanism of induction of specific un-responsiveness to allografts in animals treated with antilymphocyte serum and donor bone marrow cells, we examined the effect of donor BMC on second-set graft rejection responses caused by antidonor-sensitized spleen cells (SSC) in mice bearing donor ear skin grafts. Anti-C3H SSC were obtained from skin-grafted, ALS-treated B6AF1mice after rejection of their grafts. The second-set rejections of C3H skin grafts were assessed in B6AF1mice following adoptive i.v. transfer of 30×106SSC one day after grafting. Median survival time (MST) of C3H skin grafts in the group injected with SSC was 7 days, which is significantly lower than an MST of 11 days observed in the control group, which exhibited a first-set graft rejection response. Addition of 50×106C3H BMC to 30×106SSC abrogated the second-set rejection of C3H skin grafts (MST =11 days). This abrogation effect of BMC is strain-specific, since BMC of a third-party strain (DBA/2) failed to abrogate the second-set rejection responses caused by anti-C3H SSC. Of the different C3H lymphoid cells tested for abrogation of the second-set graft rejection, BMC were the most effective. Splenocytes were more effective than thymocytes, which showed a partial effect. Lymph node cells had no effect. Our data suggest that unresponsiveness to allografts in animals treated with the ALS/bone marrow protocol may result from the inactivation of graft-reactive cells by donor BMC.