Immediate-type hypersensitivity reactions have been studied infrequently in dogs, in part because of limited availability of antisera specific for canine IgE. A series of murine hybridoma cell lines were prepared, that produced monoclonal antibodies (MAb) with specificity for canine immunoglobulin E (IgE) and IgG. The MAb were tested for their ability to induce a reverse cutaneous anaphylaxis reaction in dog skin, to neutralize the Prausnitz-Küstner reactivity of atopic dog serum, to serve as a ligand in immunoaffinity chromatography, and to bind to IgE and other Ig subclasses in several ELISA systems. Some of the MAb produced were found to be specific for canine IgE. Other MAb recognized common or similar determinants on IgE and IgG, or on IgG and IgM, though with apparently differing affinities. Heat or acid treatment of canine IgE abolished most, but not all, of the reactivity with the anti-IgE MAb. These MAb will be useful for further study of IgE-mediated phenomena in the dog.