In the Italian Pertussis Vaccine Trial, data were collected to evaluate the persistence of anti-pertussis antibodies. A sub-cohort of 1275 children was followed for this purpose until a mean age of 21 months. An additional evaluation included pooled cross-sectional analysis of serum specimens collected for analysis of cough illnesses. Antibodies to PT, FHA and PRN were measured by ELISA using a standardized technique. With both acellular vaccines in the study (the Chiron Biocine three-component and SmithKline Beecham three-component vaccines) there was a fast and steep decrease of mean geometric titres for PT, FHA and PRN in the months immediately following vaccination. Titres were close to the detection limit 15 months after the end of primary immunization. The immunogenicity of the whole-cell study vaccine (produced by Connaught Laboratories, Inc. Swiftwater, USA) was poor as determined one month after the third dose and no antibody was detected in nearly all children 15 months after the end of vaccination.