Tetanus antitoxin levels in the sera of unimmunized lepromatous patients, often suffering from many ulcerations on their limbs due to physical and thermal trauma, but not developing clinical tetanus, were studied. Sera from 40 such patients with no history of immunization against tetanus, 35 unimmunized matched controls and 12 immunized controls were tested for the presence of tetanus antitoxin by the passive haemagglutination technique. 45% of patients, 17% of controls and all the immunized patients showed adequate serum antibody levels. Eight of the 22 leprosy patients, with no detectable antitoxin on their sera, were immunized with a single dose of tetanus toxoid and all showed sero-conversion four weeks later. 2-Mercaptoethanol (2ME) treatment of these sera showed both IgM and IgG classes of specific antibody in all groups but the level of mature antibody of IgC class was higher in the immunized group. Neutralization test further confirmed the presence of protective antibodies in these sera.