Sera from 15 patients with acute or chronic Schistosoma mansoni infection were evaluated for IgE antibodies directed against soluble cercarial, adult worm, and egg antigens. Both the antigen-induced release of histamine from passively sensitized human basophils and specific radioimmunoassays were used to detect these IgE antibodies, and determination of serum IgE levels before and after specific immunosorption permitted their quantification. While chronically infected patients made IgE antibodies to all three stages of the parasite, only egg antigens induced an appreciable IgE antibody response in acutely infected individuals. Despite the fact that patients with chronic infection had significantly greater levels of total serum IgE than patients with acute disease, the percentage of this IgE that was parasite specific was similar for both groups, ranging between 4% and 28%. An ancillary observation was the fact that soluble egg antigen can trigger basophil histamine release both through IgE-dependent reactions and through “nonimmunologic” mechanisms that require further characterization.