Indirect haemagglutination (IHA), agglutination and complement fixation tests (CFT) for Bordetella pertussis antibodies were compared on paired sera from 52 suspected cases of whooping cough and single sera from 83 children with no recent history of whooping cough. All three tests detected serotype antibodies 1, 2 and 3, but the IHA test was the most sensitive; in seven cases it was the only test to show a rise in titre. It is recommended, particularly with vaccinated children, that the serological diagnosis of whooping cough should be based upon a rise in titre. There should be a gap of at least 2-4 weeks between serum samples, depending on the age and vaccination state of the child. The CFT appears to detect a different antibody from that detected by the other two tests, and in three cases it was the only test to show a rise in titre.