The results of serological assays performed during the Italian controlled efficacy trial of two acellular vaccines and one whole-cell vaccine against pertussis are described and discussed. We examined 312 episodes of suspected pertussis disease confirmed by B. pertussis isolation, and 2862 episodes without any evidence of B. pertussis or B. parapertussis infection. Higher mean log ELISA titres for IgG to pertussis toxin (PT) were found in the acute-phase serum specimens of those children vaccinated with the acellular pertussis vaccines and particularly in the SmithKline Beecham DTaP vaccine group. These apparently anamnestic responses were responsible for the observed differences in ELISA diagnostic sensitivity exhibited by IgG to PT and by IgG to filamentous haemagglutinin (FHA). We observed minimal IgA responses to PT but vigorous IgA responses to FHA in the convalescent-phase serum specimens for the acellular pertussis vaccine groups, which contributed to the sensitivity of the serological series of assays for diagnostic purposes.