Specific tolerance can be induced in animals by transplanting hemopoietic cells across concordant species barriers. Despite the fact that the rat-mouse species combination is considered concordant, we have recently demonstrated that normal murine serum contains natural antibodies (nAb), predominantly of the IgM and IgG3 subclasses, with markedly greater binding to rat bone marrow cells (BMC) than to rat splenocytes or thymocytes. Since much greater numbers of rat BMC than of allogeneic murine BMC are required to achieve engraftment in mice, we considered the possibility that these nAbs might be responsible, and that the increased numbers of BMC might be required to absorb these nAb. To evaluate the effect of these nAb on engraftment of rat BMC in mice, we have now performed adoptive transfer studies using T and B cell-deficient severe combined immunodeficiency disease (SCID) mice as recipients. Administration of as few as 5 x 10(5) T cell-depleted rat BMC led to induction of stable xenochimerism in SCID mice conditioned with 4-Gy whole body irradiation. Rat T cells developed after a delay of several weeks, and conferred the ability to reject non-donor-type rat skin grafts, whereas donor-type grafts were accepted. Adoptive transfer of 4 ml of normal BALB/c serum led to a marked reduction in the level of rat chimerism in SCID recipients of 2 x 10(6) F344 BMC. The ability of sera to inhibit engraftment of rat BMC correlated with their cytotoxic nAb content, and the inhibitory effect of highly cytotoxic sera could be overcome by administration of large numbers of rat BMC. Thus, normal mouse serum has a limited ability to hinder engraftment of rat BMC, and this degree of resistance can be overcome by adsorption when large numbers of BMC are administered. Eliminating nAb from serum may be more difficult in discordant species combinations in recipients with functional B cells, but may likewise permit the use of BMT as a means of inducing transplantation tolerance.