Summary: Rats were immunized with 10 µg EA and alum; a low titer IgE anti-EA response was observed; subsequent infection with N. brasiliensis greatly potentiated this response. Serum from these rats was tested for its IgE content by a recently developed radioimmunoassay for rat IgE. The average serum level of IgE was 1.15 µg/ml; this level was not altered by immunization, but infection with N. brasiliensis was accompanied by an 80-fold increase in concentration of serum IgE. This increase could not be accounted for by IgE anti-EA or anti-worm antibodies. Infection with N. brasiliensis appears to have an adjuvant effect on IgE antibody responses to diverse environmental antigens as well as well as the experimentally induced IgE anti-EA response.