The in vivo efficacy of nonglycosylated recombinant human granulocyte-macrophage colony stimulating factor (rh GM-CSF) expressed in Escherichia coli was studied in cynomolgus monkeys (Macaca fascicularis). A single intravenous (IV) administration of rh GM-CSF (100 micrograms/kg) resulted in a two-fold increase in peripheral white blood cell (WBC) count with a predominance of neutrophils after 12 hr. The increased WBC count returned to the initial level within 48 hr after administration. In a study with consecutive IV administrations for 10 days, animals treated with 20 micrograms/kg/day rh GM-CSF underwent marked leukocytosis (four fold) and thrombocytosis (two fold). The increase in WBC count was due to increased number of neutrophils, eosinophils, lymphocytes, monocytes and basophils. Red blood cell count was unaffected. The leukocytosis resolved within one week after the termination of administration. A lower mean platelet volume compared to the pre-treatment level was observed in rh GM-CSF treated animals receiving 20 micrograms/kg/day for 20 days. This coincided with the elevation in platelet count.