The effects of trimetrexate following marrow transplantation were studied in a dog model. Four animals were given 9.2 Gy total body irradiation (TBI), autologous marrow, and either trimetrexate alone (0.4 mg/kg on days 1, 3, 6, and 11) or combined with cyclosporine (CSP) (15 mg/kg per day i.m. on days 1-7, then orally until day 25, then taper). All four animals engrafted normally, demonstrating that trimetrexate at this dose is tolerable. Ten additional animals were given a similar dose of TBI followed by marrow and buffy coat cells from littermate donors differing for one DLA haplotype. Trimetrexate and CSP were given as noted above. Four of the 10 animals died, one with septicemia prior to engraftment (day 14), one with intussusception (day 28), one with graft failure (day 32) and one with a tracheal abscess without graft-versus-host disease (GVHD) (day 67). Six dogs survived in excess of 100 days; one of the six developed chronic GVHD. These results are superior to those previously achieved with either methotrexate or CSP as single agents and are roughly equivalent to results achieved with a combination of methotrexate and CSP in similarly mismatched donor-recipient pairs.