Cyclosporine and prednisone were administered as graft-versus-host disease (GVHD) prophylaxis to nine patients undergoing marrow transplant from HLA matched, unrelated donors. RhGM-CSF was administered at a dose of 250 micrograms/m2 daily to all patients. The median day of neutrophil recovery to > or = 500/mm3 was Day 16. Four patients developed Grade II acute GVHD and four developed Grade III acute GVHD. One patient, who survived only 25 days, did not develop GVHD at all. One patient developed systemic infection within the first 28 days after marrow infusion. Comparison of these data to a prior series of patients undergoing bone marrow transplant (BMT) from unrelated donors who were treated with rhGM-CSF along with methotrexate and cyclosporine for GVHD prophylaxis suggests that rhGM-CSF is well-tolerated, neutrophil recovery may be earlier but the severity of GVHD does not appear reduced. Selection of the GVHD prophylaxis regimen may affect the hematopoietic response to cytokine therapy. Further trials with rhGM-CSF in patients undergoing BMT from unrelated donors are required.