Antibody responses in human sera against Bordetella pertussis during natural infection were investigated by a microplate enzyme-linked immunosorbent assay (ELISA) with a purified fimbrial haemagglutinin preparation as antigen. Significant rises of specific IgG, IgM and IgA were demonstrated in paired sera. A secondary type of antibody response was found in most children and adults. In children, the type of response correlated with previous vaccination status; there was a primary response in unvaccinated children. A survey of antibodies in the general population showed low IgG titres in a small proportion of sera from the youngest healthy children. The titres and the number of individuals with measureable antibodies increased with age. In a limited study of the effect of vaccination, significant rises of titres were demonstrated after vaccination. The ELISA test was specific for antibodies against B. pertussis except that the test also seemed to measure antibody to B. parapertussis. A comparison between ELISA and the complement-fixation test showed a good correlation between the tests only in sera from children 1-12 years old.