Serum levels of granulocyte-macrophage colony-stimulating factor (GM-CSF), granulocyte colony-stimulating factor (G-CSF) and interleukin-3 (IL-3) as well as neutrophil counts were studied on the first, second and fifth days after birth, in order to elucidate the relations between neutrophil kinetics and these hematopoietic factors. The G-CSF and GM-CSF receptors on neutrophils were also investigated in 16 healthy newborns. G-CSF and GM-CSF receptor-positive neutrophil percentages were not different from those in the peripheral blood of adult controls. The receptor densities, assessed by mean fluorescence channel number, were also similar in newborn and adult neutrophils. The mean serum concentrations of G-CSF were high (191 pg/ml) on the first day of life and gradually decreased on the second (128 pg/ml) and fifth (112 pg/ml) days. A statistically significant correlation (p < 0.05) was found between G-CSF levels and absolute neutrophil counts (ANC). Furthermore, the percentage of decrease in G-CSF levels correlated significantly with the percentage of decrease in ANC (p < 0.001). GM-CSF levels were also high, though less striking, on the first day of life (9.5 pg/ml), remained at high levels on the second (10 pg/ml), and gradually decreased on the fifth (8.8 pg/ml) day. IL-3 levels were high (110 pg/ml) on the first day of life and remained at high levels on the second (138 pg/ml) and fifth (138 pg/ml) days. We found that the IL-3, GM-CSF and G-CSF levels were elevated during the first week of postnatal life. Our findings suggest that significant changes in the levels of the growth factors are likely to be the cause of significant leukocyte blood picture changes during the first week of life. We found normal GM-CSF and G-CSF receptors in uninfected newborns.