Rabbits immunized with egg albumin produce a homocytotropic antibody. The antibody is identified by its ability to produce passive anaphylaxis in rabbit skin. The time of appearance of this antibody, its persistence and recall after booster injections depends, in part, on the route of immunization and the adjuvant employed. The physicochemical characteristics of the homocytotropic antibody obtained was similar regardless of the immunization schedule used. The anaphylactic activity of these antisera showed some heterogeneity when chromatographed on diethylaminoethyl (DEAE)-cellulose, but all fractions were inactivated by heating and absorption with a specific antisera. The anaphylactic activity could be separated from rabbit IgG and IgA, and was not blocked by absorption with antisera specific for these classes of immunoglobulins. Anaphylactic activity was completely removed by absorption with a specific antiserum which did not react with any of the known rabbit immunoglobulins. The passive cutaneous anaphylaxis titer of a rabbit serum containing homocytotropic antibody was reduced by 50% after absorption with an antisera (anti-FcND) specific for human IgE. On the basis of these distinctive physicochemical characteristics, it is concluded that rabbit homocytotropic antibody represents a unique class of rabbit immunoglobulin, analogous to human IgE.