In 1988 an outbreak of type 1 paralytic poliomyelitis occurred in Israel. Almost the entire population in the age group 0-40 years received a single dose of trivalent oral polio vaccine. We examined the serological responses to the vaccine at 2 weeks and 4 years later, in a group of 17 vaccines. Geometric mean antibody titres (GMTs) against both the type 1 epidemic and Mahoney strains had declined by about 50% from the levels found at 2 weeks after vaccination. However, they were still more than five times higher than the prevaccination levels. All vaccines had neutralizing antibody titres against both the type 1 strains of at least 1:64, well above the 1:8 titre regarded as protective. The GMTs against the type 2 and 3 strains declined to about one-third of the 2-week postvaccination levels but were also well above protective levels. These findings indicate that antibody titres against both the Mahoney and epidemic type 1 strains remained at very adequate levels over a period of at least 4 years. Thus the immunity resulting from a single booster dose of oral poliovaccine in young adults is likely to be long-lasting, a finding of particular importance for travellers on extended visits to endemic areas.