Three assays for immunoglobulin E (IgE) are compared using seven serum samples covering a wide range of values. The assays examined are the double antibody radioimmunoassay, single antibody/ammonium sulfate precipitation method and the paper radioimmunosorbent test (PRIST). In addition, values from the double antibody and single antibody/ammonium sulfate assays are compared using two different antisera to IgE. PRIST results in consistently lower values but the three assays produce results that would not have been clinically significantly different. The range of values obtained over a six-month period with aliquots of the same two samples run in duplicate twenty-three times in the double antibody assay was 1080 to 1440 I.U./ml for the Hi sample and 5.4 to 9.0 I.U./ml for the LO. Results are compared to those of a national study in which serum samples were supplied blind to research laboratories for analysis by two methods. Coefficients of variation from 35% to 73% are unacceptably large and indicate the need to establish international standards for serum IgE quantitation.