After repeated freezing, thawing and ultrasonification of the bacterial cells, the outer membrane protein (OMP) of B. pertussis was obtained through treatment with Triton X-100. The OMP was divided into 5 fractions with the molecular weights of 89000, 38000, 36000, 34000 and 30000 by Sephadex G-100 column chromatography. Two bands of proteins which had been proved to exist in B. pertussis strains I were shown in SDS-PAGE spectrum with their molecular weights estimated to be 30000 and 28000 respectively. The endotoxic activities of the OMP and its fractions were as much as 1/500 -/5000 that of the lipopolysaccharides of E.coli. The protec tive activities of the OMP and its fractions were similar to those of the corresponding weight of the whole cells of B. pertussis. The antisera against the OMP and its fractions had strong passively protective activities against intracerebral challenge with B. pertussis in mice. The results have confirmed that the method for preparing the OMP and its fractions is simple and convenient, and owing to their stronger protective activities the OMP and its fractions could be regarded as the protective antigens of B. pertussis.