Methotrexate (MTX) followed by citrovorum factor (CVF) rescue was evaluated for its effectiveness in reducing graft-versus-host disease (GVHD) in lethally irradiated dogs transplanted with bone marrow from unrelated histoincompatible donors. Animals were given no immunosuppressive therapy (group A) or a combined regimen of MTX and CVF (group AMC). These two groups were compared with a group of animals transplanted earlier given MTX alone (group AM). Animals in the AMC group lived significantly longer than the A group (p < 0.05). Engraftment rate, hematopoietic recovery and incidence of GVHD were similar in all three groups. Incidence of early deaths was significant in the AM group (p < 0.05). It is concluded that MTX combined with CVF increases survival and is an effective posttransplantation immunosuppressive regimen with minimal toxicity.