Formation of specific IgE antibodies as elicited in Sprague-Dawley rats against Ascaris antigen could be suppressed by intravenous administration both of antigen and of specific antiserum. The suppressive agent in the antiserum was shown to be antibodies of the IgG class, whereas a suppressive effect of cytophilic activity and of IgE antibodies could be outruled. Suppression of IgE response lasted the longer the more antibodies were transferred. An antibody-induced suppression was achieved when antibodies were transferred during an early period (day -3 to day +8), whereas an antigen-induced suppression took place when the antigen was intravenously administered following the antibody-sensitive period (day +8 until day +14). This is consistent with the fact that an antigen-induced suppression of IgE formation requires the presence of a certain amount of antibodies. A strictly peripheric suppression could be outruled, since with elapse of time a decreasing dose of antigen was required to induce a suppression. The results are discussed on the basis of an antigen-antibody complex-induced suppression in the IgE system and its possible central site of action.