In 1996, a seroepidemiological study was undertaken in Spain, with the main aim of estimating the population's immunity against poliomyelitis, tetanus and diphtheria. A population-based cross-sectional study was conducted, covering the population aged 2-39 years. The sample was stratified by age and rural-urban environment, and informed consent obtained to take blood specimens from subjects attending phlebotomy centres. The study included 3,932 persons and the prevalence of antibodies against all three types of poliovirus exceeded 94% across all age groups. From a high of 96% in subjects under the age of 15 years, immunity against diphtheria steadily declined to a low of 32.3% in subjects aged 30-39 years. Similarly, tetanus antitoxin concentrations indicating basic protection were present in 98.9% of the under-14 years age group; thereafter, immunity declined, until reaching 54.6% in the 30-39 years age group.