Mammalian Toll-like receptor (TLR) proteins are pattern recognition receptors that mediate cellular activation by a wide variety of bacterial products. TLR activation leads to the expression of numerous mediators of innate immunity. We used the nonpathogenic mycobacterium M. bovis BCG (BCG) to define the roles of TLR proteins in the normal development of an immune response against the bacilli. We found that TLR2-/- mice were unable to effectively control the growth of BCG in vivo, compared with normal and TLR4-/- mice. Furthermore, splenic T cells isolated from infected TLR4-/- mice could proliferate in vitro following antigen challenge, but were unable to generate a strong Th1-type response. In contrast, splenic T cells isolated from infected TLR2-/- mice could neither proliferate in vitro following antigen challenge nor generate a strong Th1-type response. Together, these studies indicate that TLR proteins participate in the development of both innate and adaptive immune responses.