We measured granulocyte colony-stimulating factor (G-CSF) levels in cord blood of 59 normal full-term neonates immediately after birth and the subsequent changes in G-CSF levels of 16 cases by our modified enzyme-linked immunosorbent assay (ELISA) for G-CSF. Ten out of 59 cases examined (17%) showed G-CSF levels in cord blood after delivery between 20 and 57 pg/ml, although in the remaining cases, the G-CSF levels were below 20 pg/ml, which was our minimal detection level. A direct relationship between G-CSF levels and white blood cell count, absolute granulocyte numbers in cord blood, gestation age or weight was not observed. Although G-CSF levels in cord blood after delivery remained below 20 pg/ml in 5 cases out of 16 tested, those in the remaining 11 cases (69%) subsequently became elevated after delivery. The peak G-CSF level in cord blood after delivery ranged from 26-364 pg/ml, and the time of it was between 4.5 and 18 h. As G-CSF level per wet weight of placenta was high (124 x 29 pg/ml), these subsequent elevations of G-CSF in cord blood after delivery may result from a gradual influx from the placenta.