A new pre-irradiation conditioning regimen which protects against radiation injury and facilitates engraftment of xenogeneic bone marrow.

Administration of a large number of xenogeneic (rat) bone marrow cells before lethal, total body irradiation in mice (preconditioning regimen) results in powerful protection from radiation damage, remarkable prolongation of their survival, facilitation of marrow engraftment and induction of rat chimerism. This protective effect is not exerted by thymus or spleen cells and does not depend on specific immune mechanisms. The preconditioning regimen is more efficient when the bone marrow cells are viable and when the cells are inoculated intravenously a few hours before irradiation.