The effects of recombinant canine stem cell factor (rcSCF) on hematopoiesis were studied in normal dogs and in dogs given otherwise lethal total body irradiation (TBI) without marrow transplant. Results were compared with previous and concurrent data with recombinant granulocyte colony-stimulating factor (rG-CSF). Four normal dogs received 200 micrograms rcSCF per kilogram body weight daily either by continuous intravenous infusion for 28 days (n = 2) or by subcutaneous (SC) injection in two divided doses for 20 days (n = 2). All dogs showed at least a twofold increase in peripheral blood neutrophil counts starting approximately 7 days after the initiation of treatment. Hematocrit level and monocyte, lymphocyte, eosinophil, reticulocyte, and platelet counts were not elevated. Marrow sections after rcSCF treatment showed panhyperplasia. The only toxicity was facial edema during the first few days of rcSCF administration, presumably caused by mast cell stimulation. Ten dogs were given 400 cGy TBI at 10 cGy/min from two opposing 60Co sources. They were given no marrow infusion and received 200 micrograms/kg/d rcSCF SC in two divided doses for 21 days starting within 2 hours of TBI. Five of the 10 dogs showed complete and sustained hematopoietic recovery and survived as compared with 1 of 28 control dogs not receiving growth factor (P < .005). RcSCF treatment allowed for hematopoietic recovery in two of seven dogs administered 500 cGy of TBI but in none of five dogs given 600 cGy of TBI. Results with rcSCF are similar to those obtained with rG-CSF. The rate of neutrophil recovery in rcSCF-treated dogs after 400 cGy TBI was not different from that of rG-CSF-treated dogs (P = .65), but the rate of platelet recovery was faster (P = .06) in the rcSCF-treated animals. Combined treatment with rcSCF and rcG-CSF after 500 cGy TBI did not result in strongly improved survival as compared with results obtained with either factor alone.