A highly purified preparation of filamentous hemagglutinin (FHA) from Bordetella pertussis was analyzed for its protein composition by gel electrophoretic methods. In this preparation of FHA the following native species could be detected by polyacrylamide gel electrophoresis (PAGE) at pH 3.2: S1 and S2 (inactive subunits or fragments); two monomers, a major form designated Ia (144K), and a minor form Ib, differing only in net charge; and three oligomeric forms, designated II (213K), III (595K) and IV (1064K). Hemagglutinating activity was associated predominantly with component Ia. PAGE of FHA after derivatization with sodium dodecyl sulfate (SDS) showed there to be three major species, designated A, C and D. According to estimated molecular weight values, A, C and D are likely to correspond to S2, Ia and II respectively. Isolated components II, III and IV yield all three SDS-species upon derivatization with SDS. Both moving boundary electrophoresis and gel electrofocusing showed hemagglutinating FHA to be a basic protein. Its apparent pI is 8.1.