A number of monovalent p-azobenzenearsonate derivatives of tyrosine elicit passive cutaneous anaphylaxis (PCA) reactions in guinea pigs sensitized with some, but not all, rabbit or guinea pig antisera raised by immunization with haptenated protein. Our data suggest that certain populations of cell-binding antibodies mediate these reactions. Reversibility of antibody-hapten interaction is indicated by failure of an eliciting injection of hapten to desensitize a site. In two hapten systems so far studied, this one and a series of dinitrophenylated compounds, certain monovalent antigens were found to be capable of eliciting cutaneous anaphylactic reactions.