We studied whether treatment of dogs with recombinant human granulocyte colony-stimulating factor (rhG-CSF), after 920 cGy total body irradiation (TBI) and transplantation of 3.3 +/- 1.0 x 10(8) bone marrow cells per kilogram from a DLA-identical littermate, accelerated hematopoietic recovery and influenced the incidence of subsequent marrow graft failure or graft-versus-host disease (GVHD). Ten animals were treated with 100 micrograms rhG-CSF/kg/d from days 1 through 10 after TBI. Results were compared with those of historical control of 14 dogs not administered rhG-CSF. Neither group of dogs received GVHD prophylaxis. The median time to recovery of 1,000 neutrophils/mm3 was 8 days for dogs administered rhG-CSF compared with 14 days in controls (logrank test: P less than .03). The median time to reach 100 monocytes/mm3 was 17 days in G-CSF-treated dogs compared with 49 days in controls (P less than .002). The median time to attain 500 lymphocytes/mm3 was 15 days versus 31 days, respectively (P less than .01). The median time to reach 20,000 platelets/mm3 was 26 versus 20 days (P = .68). Graft failure occurred in 1 of 10 G-CSF-treated dogs versus 2 of 14 controls (two-tailed Fisher's exact test: P = 1.00). GVHD was seen in 4 of 9 rhG-CSF-treated dogs compared with 1 of 12 controls (P = .12). Two G-CSF-treated dogs died of GVHD versus none of the controls (P = .17). No unusual toxicities were seen in dogs receiving rhG-CSF. In summary, rhG-CSF significantly accelerated recovery of neutrophils, monocytes, and lymphocytes after DLA-identical littermate marrow transplantation without altering platelet recovery. Graft failure was not seen more often than in controls, but there was a trend toward an increased incidence of GVHD.