Antitetanus toxin antibodies were measured, by means of the passive haemagglutination in the sera of 739 Genoese subjects, distributed in the following age classes: 1-5, 6-12, 13-20, 21-30, 31-40, 41-50, 51-60, 61-70 and greater than or equal to 71 years. It has been possible to verify that subjects of both sexes, aged less than 12 years, have high titer antibodies, particularly the female subjects show, in comparison with their male contemporaries, higher amounts of antitoxin. Besides the latter resulted at consolidated protection levels in 48% of the males, between 13 and 30 years, while for the male subjects, aged between 31 and 60 years this percentage dropped to 22 and, lastly, to 4 for those aged beyond 61 years. Furthermore for females older than 31 years, it was impossible, except exceptionally, to detect the presence of antibodies at titers greater than or equal to 0.1 I.U./ml.