We set out to investigate prospectively the levels of erythropoietin in amniotic fluid and umbilical venous blood, and to attempt to relate these to fetal haemoglobin and lactate concentrations and to pCO2 and PO2 in Rh immunised patients studied before the onset of labor. Fetal blood was obtained by cordocentesis, and amniotic fluid by amniocentesis from a consecutive series of 36 Rh immunized patients at the time of fetal blood sampling. There was a close correlation (tau = 0.357, P = 0.0001) between the concentrations of erythropoietin in umbilical venous blood and those in amniotic fluid. Erythropoietin in umbilical venous blood correlated inversely with hemoglobin (tau = 0.453, P = 0.0001), and directly with lactate concentrations (tau = 0.450, P = 0.0005). When all other variables were considered, multiple regression analysis demonstrated hemoglobin concentration to be the only variable to be related to the level of erythropoietin in umbilical venous blood taken before transfusion. When the same analysis was performed on the same variables, adding erythropoietin concentration in amniotic fluid as the dependent variable, only erythropoietin in umbilical venous blood was found to be related to the level of erythropoietin in amniotic fluid. We conclude that the erythropoietin concentration in umbilical venous blood from Rh-immunized patients before the onset of labor, is related to fetal anemia. We also conclude that erythropoietin concentration in amniotic fluid is related to that in fetal blood, thereby indicating that the fetus is an important source of amniotic fluid erythropoietin in non laboring patients.