The study investigates the role of mammalian Toll-like receptors (TLRs) in the recognition and signaling of bacterial lipopolysaccharide (LPS). Overexpression of TLR2, but not TLR1, TLR4, or CD14, confers LPS-induced activation of the transcription factor nuclear factor κB (NF-κB) in mammalian 293 cells. Mutational analysis reveals that the intracellular domain of TLR2 is essential for this response. LPS signaling through TLR2 is dependent on serum, which contains soluble CD14 (sCD14). Coexpression of CD14 synergistically enhances LPS signal transmission through TLR2, and purified recombinant sCD14 can substitute for serum to support LPS-induced TLR2 activation. LPS stimulation of TLR2 initiates an interleukin 1 receptor-like NF-κB signaling cascade. These findings suggest that TLR2 may be a signaling component of a cellular receptor for LPS.The study investigates the role of mammalian Toll-like receptors (TLRs) in the recognition and signaling of bacterial lipopolysaccharide (LPS). Overexpression of TLR2, but not TLR1, TLR4, or CD14, confers LPS-induced activation of the transcription factor nuclear factor κB (NF-κB) in mammalian 293 cells. Mutational analysis reveals that the intracellular domain of TLR2 is essential for this response. LPS signaling through TLR2 is dependent on serum, which contains soluble CD14 (sCD14). Coexpression of CD14 synergistically enhances LPS signal transmission through TLR2, and purified recombinant sCD14 can substitute for serum to support LPS-induced TLR2 activation. LPS stimulation of TLR2 initiates an interleukin 1 receptor-like NF-κB signaling cascade. These findings suggest that TLR2 may be a signaling component of a cellular receptor for LPS.