Cross-reactions between B. pertussis and 28 other bacterial species were studied by various quantitative immunoelectrophoretic methods. A sonicated B. pertussis antigen preparation and a corresponding pooled rabbit antiserum were used as reference system. Two of the B. pertussis antigens were cross-reactive with antigens from 17, respectively 19, other bacterial species mainly gram-negative species. As judged by absorption of antibodies, the degree of cross-reactivity of these B. pertussis antigens with antigens from other species was found to be in the range 25-30%. Antigens from B. parapertussis and B. bronchiseptica were found to cross-react very extensively with B. pertussis, and only 4, respectively 2, of the 44 antigens of the B. pertussis reference system could not be absorbed with antigens from these two Bordetella species.