Interaction of mouse immunoglobulins with a mast cell tumor HC was studied by the rosette technique. All mast cells formed rosettes, and this could be inhibited by IgG1myeloma proteins, less efficiently by IgG2, and not by IgA or IgM. Antigen antibody complexes formed under appropriate con ditions also inhibit, with at least 100‐fold greater efficiency than free im munoglobulin. Antisera containing reaginic activity also inhibit rosettes in the presence of antigen, but on heating the sera, a concomitant loss of reaginic activity and rosette inhibition occurs. It is proposed that there is a specific receptor on the mast cell surface for IgG1and reaginic immunoglobulins, and that binding to this site in the form of a complex of correct antigen ratio, will initiate the anaphylactic proces