When the three serotypes of Bordetella pertussis (types 1,2,3; 1,2 and 1,3) were labelled with agglutinins and protein-A gold, agglutinogen 1 was found on fimbriae and on the cell surface of types 1,2,3 and 1,2 but on the cell surface only of non-fimbriate type 1,3 organisms. In contrast, agglutinogen 2 was located on fimbriae only. Agglutinogen 3 was not labelled. When protein-A gold was replaced by immunoglobulin-G gold, agglutinogen 3 was found on the cell surface only, even of fimbriate bacteria of type 1,2,3. The implications of these findings for acellular vaccines are discussed.