An enzyme-linked immunosorbent assay (ELISA) and bacterial agglutination (BA) method for determining the presence of antibodies to Bordetella pertussis were compared on serum samples from 21 patients with whooping cough and their 76 family members. The overall diagnostic agreement between the two methods was 77%. The data for BA-detected antibodies correlated best with IgG and IgA antibodies to B. pertussis. All of the culture-positive patients showed serologic positivity in both assays during the follow-up. Pertussis was diagnosed by ELISA in most cases from the first serum sample. Both methods proved to be good diagnostic aids in culture-negative patients, although the value of BA is more retrospective because of the need for paired sera. The kinetics of IgM, IgA, IgG, and agglutinating antibodies to B. pertussis is presented.