Peritoneal mast cells from immunized rats can form rosettes with antigen-coated sheep red blood cells. The receptor responsible for this active rosette formation is Ig[immunoglobulin]E cytophilic antibody. Rosettes are inhibited by previous contact of mast cells with antigen, or with anti-IgE antiserum. The kinetics of mast cell rosettes following a primary immunization with ovalbumin and Bordetella pertussis vaccine is similar to the kinetics of the reaginic antibody response. A reaginic serum can induce passive rosette formation. There is no correlation between cell-bound and circulating IgE.