High serum IgE levels were found in eighty-nine patients with hydatid disease. Specific IgE antibodies to E. granulosus hydatid fluid or to a pure E. granulosus antigen were isolated using immunoabsorbents and quantified by a radioimmunoassay. The procedure made it possible to quantify, for the first time, the concentration of IgE antibodies in units per millilitre. Significant levels of IgE antibodies were demonstrated in twenty-three out of thirty-nine patients, by the use of standardized hydatid fluid antigens. The amount of IgE antibodies to E. granulosus was not related to serological findings. However, a highly significant correlation was obtained between levels of total serum IgE and IgE antibodies. A pure antigenic fraction prepared from hydatid fluid and known to be specific for E. granulosus was shown to be the most potent antigen reactive with IgE antibodies. The relationship of IgE antibodies to the viability of the hydatid cyst and to the serum IgE level is discussed.