Development of antibody titres in non-vaccinated children with whooping cough of different duration (all confirmed by positive culture) were investigated by ELISA using lymphocytosis promoting factor (LPF, pertussis toxin), filamentous haemagglutinin (FHA), 69 kDa protein and lipopolysaccharide (LPS) as antigens. The antibody responses occur in three different patterns: Firstly, the LPF antibody response develops very quickly starting with the first day of clinical cough with all three classes, IgG, IgM and IgA appearing simultaneously; LPF antibody appears to be a dominant feature. Secondly, FHA and 69 kDa antibodies appear, starting as IgM with the shift to IgG and IgA later. The third pattern is represented by LPS antibody, the IgA appearing early, but with IgM predominant. Higher titres of IgG reacting with LPS were observed in vaccinated children. Transplacental transfer of antibody was also studied. All antibody titres determined in maternal blood and cord blood were proportional except for anti-LPS antibody which was retarded. Most IgG antibody was IgG1 subclass; surprisingly the 69 kDa antibody consisted of a mixture of approx. 90% IgG1 and 10% IgG4.