In a factory population the occurrence of reactions to tetanus toxoid was recorded after 6740 injections. The incidence of general reactions was 0.3% and of local reactions 2.6%. The local reaction rate to the first injection of the basic immunization course was 0.9%, to the second injection 2.7%, and to the third injection 7.4%. To booster injections the rate was 1.6%. The local reaction rate was appreciably higher in women than in men - 14.4% and 5.7% respectively in the case of the third injection - and the incidence among women increased with age.Tetanus vaccine containing 10 Lf of toxoid caused fewer reactions than one containing 20 Lf, but a reduction in the content of aluminium adjuvant did not affect the reaction rate.Almost all reactors were found to have a satisfactory serum antitoxin concentration at the time of the reaction or developed a satisfactory immunity within 1-6 months.Skin tests were made in 32 hypersensitive patients. Neither the diluent, thiomersal preservative, nor the culture medium appeared to be responsible for hypersensitivity. The degree of hypersensitivity elicited by a special highly purified toxoid was only very slightly less than that elicited by the commercially pure toxoid. It is suggested that reactions are largely due to the toxoid antigen itself rather than to impurities or other components of the vaccine.