Anthrax, a disease caused by Bacillus anthracis, affects animals and humans. Because the inert spore is the infectious form of the organism that first contacts the potential host, the interaction between the host and spore exosporium is vital to the initiation of disease. Here, we demonstrate that the integrin Mac-1 is essential for the recognition of the major exosporium protein BclA by phagocytic cells. Expression of Mac-1, but not p150/95, in CHO cells markedly enhanced infection with Sterne strain of B. anthracis spores (WT spores). Conversely, CD11b-/- macrophages demonstrated a significant decrease in spore uptake when compared with macrophages from normal C57BL/6 mice. However, when CD11b-/- macrophages were infected with ΔbclA spores, spore ingestion was no different from their C57BL/6 counterparts. ΔbclA spores were also efficiently internalized by all CHO cell lines tested, independently of Mac-1 expression. Taken together, these results show that there is an alternative Mac-1-independent pathway involved in spore uptake that is unmasked only in the absence of BclA. Survival studies, using C57BL/6 and CD11b-/- mice, revealed that CD11b-/- mice are more resistant to infection with WT but not ΔbclA spores. Our experiments also show that ΔbclA spores are more virulent than WT spores in C57BL/6 and A/J mice. Overall, our data indicate that the Mac-1/BclA interaction may play a major role in B. anthracis pathogenesis by promoting spore uptake by professional phagocytes and subsequent access to a favorable niche for transport, germination, and outgrowth in lymphoid tissues. © 2008 by The National Academy of Sciences of the USA.