Studies of the myeloablative regimens capable of permitting successful BMT across MHC barriers in miniature swine have been performed. To minimize graft-versus-host disease (GVHD), engraftment was studied in the F1-->P combination (i.e., MHC homozygous ["parental"] swine receiving bone marrow from one-haplotype matched MHC heterozygous ["F1"] donors). Animals given total body irradiation (TBI) up to 1100 cGy, 10 cGy/min, in a single dose failed to engraft. Increasing the dose rate led to unacceptable extramedullary toxicity without improving engraftment. Eleven different fractionated TBI regimens were tested in this F1-->parent model. At all of the dose rates tested, a total dose of less than 1000 cGy was insufficient for engraftment, and a total dose of 1400 cGy led to unacceptable toxicity. Between these extremes, a window was defined in which engraftment could be obtained without unacceptable extramedullary toxicity utilizing 2 equally divided fractions of TBI delivered 24 hr apart. The addition of 50 mg/kg cyclophosphamide i.v. to fractionated TBI (1150 cGy total dose [500 + 650]) also permitted engraftment, with decreased incidence of interstitial pneumonitis as compared to fractionated TBI (1300 cGy total dose [650 x 2]). Both of these regimens were also confirmed to permit engraftment between heterozygous donors and recipients sharing a single common haplotype ("F1-->F1"). The regimen of 1300 cGy (650 x 2) also permitted engraftment in completely MHC mismatched BMT, but with subsequent death from GVHD. These studies of the myeloablative regimens permitting engraftment across defined MHC barriers in miniature swine provide a basis for further studies of allogenic BMT and GVHD in this large animal preclinical model.