The seroprevalence of antibodies to measles, mumps, and rubella viruses (MMR) was determined in 1498 subjects in Catania, Italy, ranging in age from 1 month to 25 years. The study population was divided into seven age groups and screened by enzyme immunoassay using microquantities (10 microl) of whole blood collected by fingerprick on filter paper discs. The results showed that seroconversion for measles (87.6%) and mumps (73.2%) occurred between 6 and 10 years of age. The seroprevalence of antibodies to rubella virus increased slowly through the age groups, reaching the highest rate (93.3%) between 16 and 20 years of age. Passively transmitted maternal antibodies to mumps and rubella were absent in babies between 5 and 8 months of age, and a few cases positive for measles antibodies were found among babies 6 and 7 months of age. The enzyme immunoassay was demonstrated to be suitable for low-cost large-scale screening for MMR immunity. The rate of vaccine failure was also evaluated and found to be 9.5% for the measles virus, 12.9% for the mumps virus and 0.0% for the rubella virus.