The commercially available hemagglutination inhibition (HAI) assay kit for erythropoietin (ESF) was compared with the fetal mouse liver cell (FMLC) bioassay. No correlation was obtained ESF levels determined by both methods in a variety of pathologic sera. The HAI kit showed a great batch variability. Significant immunoreactivity was found in those fractions of a normal human serum and a human urinary ESF preparation that were not active in the FMLC bioassay. A very poor recovery of immunoreactivity was found when the international reference preparation for erythropoietin (second IRPE) was added to a normal human serum.