OBJECTIVE To determine fetal serum and amniotic fluid (AF) levels of interleukin (IL)-3, IL-6, granulocyte-macrophage colony-stimulating factor, stem cell factor, and erythropoietin, and to explore the relationship between cytokines and hemoglobin concentration, white blood cell count (WBC), and platelet count in fetuses affected by Rh immunization. METHODS Thirty-four consecutive Rh-immunized patients in gestational weeks 19-33 were included. All patients were investigated by funipuncture and 13 by amniocentesis. The levels of IL-3, IL-6, granulocyte-macrophage colony-stimulating factor, stem cell factor, and erythropoietin were estimated using commercially available immunoassays. RESULTS There was a significant correlation between erythropoietin concentrations in fetal serum and AF (r = 0.54, P < .05), whereas none of the other cytokines showed a positive correlation between these two compartments. Fetal serum contained higher concentrations of IL-3, granulocyte-macrophage colony-stimulating factor, stem cell factor, and erythropoietin compared with AF. In contrast, the IL-6 level was significantly higher in AF compared with fetal serum (P = .002). Erythropoietin and IL-3 levels were both negatively correlated with fetal hemoglobin concentrations (r = -0.75, P = .02, and r = -0.67, P = .045). The fetal WBC correlated significantly with the fetal serum concentration of granulocyte-macrophage colony-stimulating factor (r = 0.38, P = .04). CONCLUSIONS Human fetuses with anemia due to erythrocyte immunization exhibit an increased production of erythropoietin and IL-3. Other studied cytokines (such as stem cell factor, granulocyte-macrophage colony-stimulating factor, and IL-6) did not correlate with the degree of fetal anemia. Among the studied cytokines, only erythropoietin showed a positive correlation between fetal serum and AF.