A new factor that inhibited phagocytosis to opsonized targets and chemotaxis of PMN was extracted from B. pertussis cells, and named PMN-inhibitory factor (PIF). Cells in phase I produced 10 times more PIF than those in phase III, and like other phase I-associated components--the hemagglutinin, the histamine-sensitizing factor and agglutinogens--PIF showed degenerative, phenotypic variation during in vitro culture of phase I bacteria. PIF was partially purified by four steps, including adsorption chromatography on Dansyl-aminononamethylene Sepharose. The resulting fraction was heterogeneous but showed little histamine-sensitizing and cytotoxic activities and was free from LPS, the hemagglutinin and a leukocyte agglutinin. The inherent resistance of B. pertussis cells, in either phase I or III, as demonstrated also in the present study, and PIF-mediated defiance against immunological defense mechanism may constitute a complex host-parasite relation in experimental infections with B. pertussis.