The present study demonstrates that mercuric chloride (HgCl2) induces a striking increase of total serum IgE in Brown Norway (BN) rats. Values up to several milligrams of IgE per milliliter of serum were observed. No antibody specificity was demonstrated for these IgE. Mercuric chloride also potentiated a specific anti-ovalbumin IgE response when the rats were immunized with ovalbumin. The kinetics of the response to HgCl2 was different from that to an antigenic stimulus alone: total IgE increased together with the potentiated anti-ovalbumin IgE antibody response to reach maximum values about 14 days after initiation of HgCl2 injections. The potentiated IgE antibodies represented only an insignificant fraction of total IgE. All these findings were observed in BN but not in Lewis rats. These data show analogies with those reported after parasitic infection in the rat and suggest similar mechanisms of action.