Techniques are described in detail for the RIA of human Ep in unextracted plasma or serum. With 100 microliters of sample, the essay is sensitive at an Ep concentration of approximately 4 mU/ml, and when required, the sensitivity can be increased to 0.4 mU/ml, a range considerably less than the concentration observed in normal human beings. This is approximately 100 times more sensitive than existing in vivo bioassays for this hormone. Studies concerned with the validation of the Ep RIA show a high degree of correlation with the polycythemic mouse bioassay. Dilutions of a variety of human serum samples show a parallel relationship with the standard reference preparation for Ep. Validation of the RIA is further confirmed by observations of appropriate increases or decreases in circulating Ep levels in physiological and clinical conditions known to be associated with stimulation of suppression of Ep secretion. Significantly different mean serum concentrations of 17.2 mU/ml for normal male subjects and 18.8 mU/ml for normal female subjects were observed. Mean plasma Ep concentrations in patients with polycythemia vera are significantly decreased, and those of patients with secondary polycythemia are significantly increased as compared to plasma levels in normal subjects. These results demonstrate an initial practical value of the Ep RIA inthe hematology clinic, which will most certainly be expanded with its more extensive use.