The immunity state to poliovirus types 1, 2, and 3 of a population aged 2 to 75 years was determined by examining 274 sera collected in the Venice mainland (North-East Italy). Altogether, the neutralizing antibody prevalences (at a titre > or = 1:2) for poliovirus 1, 2, and 3 were 99.0 percent (geometrical mean titres [GMT]: 72.1), 99.6 percent (GMT: 95.9) and 98.2 percent (GMT: 17.3), respectively, and all the age groups also showed very good levels of humoral immunity. High antibody titres (> or = 1:256) to one or more types of poliovirus were demonstrated in older age groups also, possibly indicating exposure to natural polioviruses or contact with vaccine strains. Although probably all the older subjects had practically acquired their antibodies as a result of natural infection and those under 30 through vaccination, these results indicate that the humoral immunity against poliomyelitis in our population is satisfactory, and the maintenance of such good protection level depends on an effective immunization program.