Immunization of Hartley strain guinea pigs with large doses (1 to 5 mg) of an ethylchloroformate polymerized Ascaris suum antigen, and immunization subsequently followed by infection with living second-stage larvae of the parasite, resulted in the production of both IgE (γE) and IgG1 (γG1) homocytotropic antibodies in high titer. Reaginic (PK) activity was assayed after a 6-day latent period and sedimented in sucrose density gradients at 8.3 ± 0.3S and 10.7 ± 0.3S. Both the 8S and 11S components were also demonstrated by antiglobulin enhancement of hemagglutination with specific anti-guinea pig IgE and anti-human IgE sera. The 11S reaginic antibody had a greater skin-sensitizing activity than the 8S component on the basis of the relative quantities detected following antiglobulin enhancement. The reaginic activity also was separated into different sized components by Sephadex G-200 gel filtration. Guinea pig PK activity was not absorbed by rabbit anti-guinea pig IgM (γM) or anti-IgG2/G1 (γG2/G1) sera. The IgG1 (γG1) (passive cutaneous anaphylatic (PCA) activity, assayed after an 18-hr latent period, was removed by the anti-IgG2/G1 (γG2/G1) serum. Reaginic activity was absorbed by rabbit anti-guinea pig IgE (γE) sera, and also by monospecific anti-human IgE (myeloma PS) serum. The anti-guinea pig IgE and anti-human IgE sera did not absorb guinea pig IgG1 PCA activity. Both monomeric and polymeric reaginic activities were absorbed by rabbit anti-guinea pig IgE serum. These experiments establish that guinea pig reaginic (IgE (γE)) antibody is the analogue of human reaginic antibody of the IgE class, and that the guinea pig and human IgE-globulins share common class-specific antigenic determinants.