In vivo biologic effects of the polymorphonuclear leukocyte-inhibitory factor (PIF) of Bordetella pertussis were tested by using two experimentally induced. inflammatory processes in mice. The intravenous injection of a partially purified extract from phase I bacteria strongly inhibited the glycogen-induced peritoneal infiltration of polymorphonuclear leukocytes (PMN) and the Arthus reactions, whereas little inhibitory activity was found in the extract from phase III bacteria. The activity was localized in the outer membrane of phase I bacteria, as was the in vitro PIF activity, and the two activities gave the same behavior in DEAE-cellulose chromatography.Therefore the observed suppression of inflammatory processes in mice is probably due to the inhibitory action of PIF on the function of PMN in vivo.Earlier studies have shown that Bordetella pertussis vaccine renders its recipients more susceptible to a variety of bacterial (1, 3, 5, 6) and viral (13) infections. These organisms might produce an active component which depresses the host's defense mechanism. Thus Levine and Sowinsky (9) made an interesting observation that injection of pertussis vaccine inhibited the migration of monocytes into thermally coagulated, necrotized loci in the brains of experimental mice. The polymorphonuclear leukocyte-inhibitory factor (PIF), which has been shown to be produced by virulent strains of B. pertussis and to interfere with the phagocytic and chemotactic functions of human polymorphonuclear leukocytes (PMN) in vitro (4,14), may also be related to the deterioration of the host's defense. Therefore we designed experiments to see if PIF could also block the PMN-mediated inflammatory responses in vivo by the use of the experimental systems of Arthus reactions and glycogen-induced peritoneal infiltration of PMN in the mouse. The results indicate that this may indeed be the case.