Bioassays for human erythropoietin are cumbersome, time-consuming, and insensitive. The purification of human erythropoietin (EP) had provided small quantities of highly bioactive EP (approximately 70,000 U/mg) required for the development of an EP radioimmunoassay (RIA). The RIA for EP described in this investigation, can detect 5 mU/ml of EP in the assay tube; the serum concentration of EP in normal individuals ranged from less than 18 to 81 mU/ml with a mean value of 29 mU/ml. In contrast, nine patients with severe aplastic anemia had markedly elevated serum EP concentrations with a mean value of 3,487 mU/ml, range 984--6,434 mU/ml. In this RIA, patients with Polycythemia vera had consistently undetectable EP concentrations, less than 18 mU/ml. Eleven patients with chronic renal failure had a higher mean serum EP concentration (40.5 mU/ml) than normal individuals, but the range (less than 18-115 mU/ml) overlapped that of normals. By immunologic and gel chromatographic criteria, EP measured in serum was similar to standard urinary EP. The EP immunoassay that we have developed has sufficient sensitivity and specificity not only to quantitate the elevated serum EP levels found in aplastic anemia but also to discriminate decreased from normal serum concentrations of EP in most circumstances. This simple, reliable RIA has provided the necessary framework upon which to increase our understanding of the importance of EP in hematopoiesis.