Abstract: A sensitive and reliable sandwich enzyme linked immunosobent assay(ELISA)has been developed for determination of concentration of recombinant human granulocyte-macrophagecolony-stimulating factor(hGM-CSF).The assay is quantitative between 0.39~12.5 ng · ml^-1)forbacterially synthesized hGM-CSF in rat serum and urine. The methed was shown to be highly specificand did not significantly alter the determination when adding some potential interfering substances.After single sc injection of hGM-CSF 50， 100 or 200 μg·g(-1), a high GM-CSF level was detectedabout 15 min in rat serum, the highest level of hGM-CSF was two apparent phases with half-lives PT_1/2α of 0.72,0.70,0. 80 h and T_1/2β of 8.77,8.87 and 5.58h.A detectable urinary excretionoccured after sc injection of hGM-CSF 200 μg·kg(-1),but the total urinary excretion of unchangedhGM-CSF was verv low.