The pathogenesis of infection by Bacillus anthracis has been the subject of many investigations, but remains incompletely understood. It has been shown that B. anthracis spores germinate in macrophages and perhaps require this intracellular niche to germinate in vivo before outgrowth of the vegetative organism. However, it has also been reported that macrophages are sporicidal in vitro. In our in vivo model, macrophages were depleted from mice by either silica treatment or treatment with liposome-encapsulated dichloromethylene disphosphonate (Cl(2)MDP), and the animals were infected parenterally with virulent ungerminated B. anthracis (Ames strain) spores. The mice in which macrophages had been depleted were killed more rapidly than untreated mice. In addition, augmenting peritoneal populations of macrophages with cultured RAW264.7 cells partially protected mice from disease, increasing the survival rate in a dose dependent relationship. Alveolar macrophages were depleted by intranasal instillation of liposome-encapsulated Cl(2)MDP. The animals with normal alveolar macrophage numbers had significantly greater survival rates after inhaling B. anthracis spores than the macrophage-depleted mice. These findings do not preclude the observations that macrophages provide a site permissive for spore germination, however, these data suggest that macrophages do play an important role in limiting and/or clearing a B. anthracis infection.