Booster immunization with tetanus/diphtheria toxoid has now been used for several years by a number of clinical immunology laboratories throughout the country to assess B cell related immunologic profiles in patients and control normal subjects. We have recently developed a sensitive ELISA assay for IgM and IgG in vitro specific antibody responses against tetanus toxoid. This will allow us to carefully and precisely monitor the evolution and immunoregulation of the human B cell responses to this recall antigen. This is of particular importance and relevance to us in our ongoing studies of a number of diseases characterized by abnormalities of immunologic reactivity, including those treated with various regimens of immunosuppressive agents.