The influence of killed Bordetella pertussis cells (B.p.) on the cell-mediated resistance of mice against infection with virulent germs of Listeria monocytogenes has been studied. Resistance of mice was decreased, when 3 X 10(9) B.p. were injected 1 day before, simultaneously with or 1 day after infection, resulting in augmented amounts of viable Listeriae recovered from the spleens 3 days after infection (figure 1). The LD50 was strongly reduced (Table 1). Transfer of immune spleen cells to recipient mice, which had been treated 1 day previously with 3 X 10(9)B.p., did not support resistance definitely (Table 2). Therefore, it can be concluded that probably the macrophage system was impaired just after B.p. injection. When, however, B.p. were given several days before infection, resistance was increased. A maximum of resistance enhancement was seen 7-14 days after B.p. treatment. Thereafter, this beneficial effect gradually decreased but persisted for at least 67 days (figure 1). This resistance enhancing effect of B.p. was surely not due to adjuvant effect of B.p. on the T-lymphocyte-mediated immune reaction to Listeriae, since in B.p.-pretreated mice the development of immunity during the primary infection to a secondary listeric infection has even been lacking (Table 3). It is more likely that the macrophage system was stimulated at this time by B.p. In mice treated 7 days prior to infection the elimination of Listeriae from the spleens was supported from the very beginning of the infection (figure 2).