Since cases of lepra reaction following smallpox vaccination and BCG vaccination had been reported the effect of tetanus immunisation on leprosy patients (whether it may provoke a lepra reaction or not) was studied. Three doses of purified tetanus toxoid (one ml initially, one ml after six weeks and one ml after six months) were given to 357 leprosy patients and 60 patients living in the same environ were followed as controls. The antibody response following immunisation was followed in six lepromatous leprosy patients using toxin antitoxin neutralisation test at the Lf/1000 level in mice and in three of them the antibody titre of leprosy patients rose to satisfactory level. The number of lepra reactions in these patients was monitored for nine months (two months before vaccination, during the six months period of vaccination and one month after the last dose of vaccine). There was no significant rise in the number of patients with reaction following the vaccination.