Erythropoietin is now accepted as the hormone controlling red blood cell production. A radioimmunoassay specific for human erythropoietin has been developed in this laboratory using purified radioiodine labeled human erythropoietin and an anti-erythropoietin anti-serum prepared in rabbits. Although use of this radioimmunoassay in serum from normal individuals has resulted in data which are consistent with bioassay data, discrepancies have appeared when the radioimmunoassay has been used in certain clinical situations. In particular, very high values are observed in the serum of anephric patients by radioimmunoassay, whereas, no erythropoietin is detectable by bioassay of the same serum. A significant portion of this proposal is concerned with physicochemical, biological and immunological characterization of the material responsible for this high immunoactivity. We also propose to reevaluate systematically the production and characterization of antibody to erythropoietin with the aim of developing methods for the production of high avidity, monospecific erythropoietin antibody which potentially can be applied to erythropoietin radioimmunoassay throughout the medical community. In the development of the radioimmunoassay for human erythropoietin we have gone to considerable effort to obtain labeled erythropoietin free of labeled contaminants. Labeled erythropoietin, prepared in this manner, will be used in a search for specific erythropoietin binding proteins in erythropoietic tissues, the aim of which is the development of a radioreceptor assay for erythropoietin. Such a radioreceptor assay when used in conjunction with the radioimmunoassay for erythropoietin should give a new dimension to the study of hematological disease. Also a radioreceptor assay should have the potential of measuring erythropoietin levels in normal laboratory animals, The knowledge gained from this investigation will add significantly to our understanding of the biogenesis and mechanism of action of erythropoietin and to the role of this hormone in normal and diseased states in humans.