SUMMARYPassive cutaneous anaphylactic (PCA) reactions are shown to be elicitable with an H-2 antigen-antibody system in mice. The solubility and innocuity of the antigenic preparations are important for the outcome of the reaction. H-2 anaphylactic activities can be separated from complement-fixing activities. The responsible antibodies were contained in either transplantation immune sera or serum fractions. They are related to electrophoretically fast migrating immunoglobulins of 7 S IgG class, reaginic antibodies not being involved in the reaction. IgG1antibodies are the best candidates as mediators of the reaction. However, a fast migrating portion of IgG2has not been excluded as a possible candidate. The use of this technique for analyzing the biological properties of transplantation immune sera is discussed.