Transforming growth factor-β–activated kinase 1 (TAK1) has been linked to interleukin 1 receptor and tumor necrosis factor receptor signaling. Here we generated mouse strains with conditional expression of a Map3k7 allele encoding part of TAK1. TAK1-deficient embryonic fibroblasts demonstrated loss of responses to interleukin 1β and tumor necrosis factor. Studies of mice with B cell–specific TAK1 deficiency showed that TAK1 was indispensable for cellular responses to Toll-like receptor ligands, CD40 and B cell receptor crosslinking. In addition, antigen-induced immune responses were considerably impaired in mice with B cell–specific TAK1 deficiency. TAK1-deficient cells failed to activate transcription factor NF-κB and mitogen-activated protein kinases in response to interleukin 1β, tumor necrosis factor and Toll-like receptor ligands. However, TAK1-deficient B cells were able to activate NF-κB but not the kinase Jnk in response to B cell receptor stimulation. These results collectively indicate that TAK1 is key in the cellular response to a variety of stimuli.