A large amount of M-CSF and G-CSF exists in human amniotic fluid and both are considered to have some physiological affect on maintaining pregnancy. We therefore examined the source of M-CSF and G-CSF found in the amniotic fluid. The average level of M-CSF in the amniotic fluid of patients without complications was 17.3 +/- 8.5 ng/ml and that of G-CSF 1.85 +/- 1.72 ng/ml, both being high values. In neonatal urine, the average level of M-CSF was also very high, 144.3 +/- 97.0 ng/ml, but that of G-CSF was below the determination limit of 60 pg/ml. Immunohistochemical staining indicated that production of M-CSF and G-CSF was localized in the epithelial cells of fetal membrane. On the basis of the above observations, M-CSF was found to derive from neonatal urine and the epithelial cells of fetal membrane, and G-CSF from the epithelial cells of fetal membrane.