The study of antigen specific IgG subclass distribution during disease, or during any other natural or artificial immunisation, can provide useful information on the kind of the immune response and the expected levels of protection. This is particularly true for diseases, such as pertussis in which the mechanisms underlying specific defence are still not completely understood. An investigation was therefore performed to evaluate the IgG subclass response to pertussis toxin (PT) in sera from 89 healthy vaccinated children and 131 vaccinated or unvaccinated children convalescent after a confirmed B. pertussis symptomatic infection. Antibody titres were expressed in arbitrary ELISA units/ml, and statistical analyses were performed. In unvaccinated convalescent children IgG1 and IgG3 were prevalent whereas in children immunised with two different acellular pertussis (aP) vaccines, both healthy and convalescent, IgG1, IgG2 and IgG4 antibodies were mainly produced. Maintenance of the same anti-PT antibody response pattern in healthy acellular pertussis vaccine recipients and in vaccinated children who later acquire the disease is an interesting result indicative of the priming effect induced by these vaccines in the direction of a relatively higher Th2 cell-polarisation of the immune response.