One of the major interests of this laboratory is the characterization of the cellular subpopulations involved in the triggering and immunoregulation of B lymphocytes to produce antibodies. We are assessing the responses of individuals to produce antibodies against newly encountered antigens such as keyhole limpet hemocyanin (KLH) (approved project number 77-I-0130) and propose to assess the amnestic responses to the previously encountered antigen, tetanus toxoid. The purpose of this protocol is to add this recall antigen to our repertoire of antigens used to evaluate the immune response in man.