The specific IgE response that appears in subjects immunized with tetanus toxoid does not induce hypersensitivity reactions at subsequent immunizations. The type I immune response, therefore, was studied both in vivo and in vitro, in 11 subjects who had specific IgE antibodies for tetanus toxoid. The results showed that: 1. the specific IgE antibodies are heterogeneous regarding their affinity for the mast cell and basophil receptors; 2. the specific IgG antibodies for tetanus toxoid, at serum concentrations, are not able to interfere with the in vitro specific basophil degranulation; 3. in the PEG precipitate there are aggregates of specific IgE antibodies for tetanus toxoid. In vitro, these molecular aggregates are not able to sensitize the basophil cells.