Three radioimmunoassay techniques [double antibody, Sepharose-bound antibody and bromacetylcellulose (BAC)-bound antibody] were compared. These three methods were used to measure the immunoglobulin E levels in sera of patients with schistosomiasis with very high IgE levels, in sera from normal adults, and in sera that were presumed to have very low IgE levels, including sera from immunodeficient patients and from a variety of non-primate species. There was, in general, good agreement among the three methods when the IgE concentration was elevated, as in patients with schistosomiasis and in some normal adults. In contrast, very significant differences in the estimates of the IgE concentration of the same serum were observed with the different techniques when the serum IgE levels were low. IgE was very low or undetectable in sera of many patients with immunodeficiency when studied by the double antibody technique, but these IgE deficiencies were often missed when the Sepharose or BAC techniques were used. For example, the IgE levels were found to be less than 15% of normal for 80% of the patients with ataxia telangiectasia studied by the double antibody technique, but IgE levels appeared to be normal when assayed by the Sepharose and BAC techniques. The IgE levels determined by the Sepharose method in patients with immunodeficiency were intermediate between the lower double antibody and the higher BAC values. Similarly, sera of various non-primate species were found to inhibit significantly the binding of radiolabeled IgE to the insoluble anti-IgE antibody and gave significant appearent IgE concentrations when the BAC or Sepharose techniques were used, but not when the double antibody technique was used. Thus, the BAC and, to a lesser extent, the Sepharose technique appeared to give falsely high IgE estimates when the levels of human IgE in the serum were low. The double antibody radioimmunoassay appears to be the method of choice for the measurement of serum IgE because it has greater sensitivity and reproducibility, and especially because it gives meaningful estimates for IgE in the critically important low IgE range observed with immunologic deficiency.