Passive transfer of immunity to with pooled antiserum from immune rats was neutralized by intravenous injection of small amounts of a saline extract of adult worms. This inhibition of protection was associated with systemic anaphylaxis and appeared to result from the neutralization of protective antibodies. 2. Serum from infected rats was fractionated by G-200 Sephadex gel-filtration. Reaginic antibodies were shown to be intermediate in molecular size between 7S and 19S globulins in sera from both singly and multiply infected animals. In immunoelectrophoresis they migrated with fast immunoglobulins but could not be related to either IgG or IgA rat immunoglobulins. The same serum fractions gave both homologous passive cutaneous anaphylaxis (PCA) and systemic anaphylaxis. 3. Blocking antibodies were found both in the 7S and 19S fractions after separation on G-200 Sephadex. These antibodies were found in sera from rats immunized with worm extracts as well as in sera from singly and multiply infected animals. 4. The saline extract of adult worms was fractionated on G-200 Sephadex. The isolated antigenic material (allergen) for both homologous PCA and systemic anaphylaxis seemed to be a protein with a molecular weight of approximately 12,000–17,000.
