Ultracentrifugation was used to separate IgG from IgM in serum samples from children after pertussis vaccination or infection and the fractions were examined by indirect haemagglutination (IHA) and complement-fixation (CF) tests. IHA detected pertussis-specific IgM and IgG but CF detected only IgG which appeared to be of a different subclass from the IHA IgG. The IgM titres were higher after infection than after vaccination but the reverse tended to apply to the IgG titres. Little IgM or CF IgG was detected 5 months after a dose of vaccine, but the IHA IgG persisted longer. Vaccinated children who were subsequently infected showed IgM and CF IgG responses similar to those of unvaccinated, infected children but the IHA IgG titres reached much higher levels.