Previous results from our laboratory demonstrated that purified lymphocytosis-promoting factor (LPF), a protein toxin from Bordetella pertussis, inhibited the migration of murine macrophages in vitro. The current study examined the in vivo effects of LPF on mononuclear phagocyte circulation and response to an inflammatory stimulus. Intravenous injection of mice with 200 ng of LPF produced a prolonged monocytosis which peaked with a fivefold increase on day 5 after injection. LPF (200 ng) also inhibited by more than 75% the increase in peritoneal inflammatory macrophages induced by intraperitoneal injection of thioglycolate broth, phytohemagglutinin, or paraffin oil. The inhibition was significant when thioglycolate was given 1 h or 2 or 4 days after LPF but not when thioglycolate was given 2 or 4 days before LPF. The LPF-induced monocytosis on day 5 after injections was not altered by the intraperitoneal injection of thioglycolate broth. The leukocytosis-promoting and macrophage-inhibiting properties of LPF were the same in N:NIH(S) and C3H/HeJ mice. Treatments of LPF that reduced the leukocytosis-promoting effect of LPF also reduced the ability of LPF to inhibit the macrophage response. LPF doses sufficient to induce leukocytosis (greater than or equal to 25 ng) significantly inhibited the thioglycolate-induced increase in peritoneal macrophages. The results indicate that coincident with an LPF-induced monocytosis is a reduction in the number of mononuclear phagocytes at a site of inflammation. An in vivo inhibition of mononuclear phagocyte migration would explain both effects of LPF and is consistent with the in vitro inhibition of macrophage migration by LPF.