Tripalmitoyl-S-glyceryl-l-Cys-Ser-(Lys)4 (Pam3CSK4) is a highly conserved molecular motif found in various classes of lipoproteins. The requirement for leukocyte to respond to synthetic Pam3CSK4 were studied. Pam3CSK4 primed neutrophils for a respiratory burst in a serum-dependent manner. Pam3CSK4 upregulated CD11b, CD14, and cytochrome b558, and downregulated Leu-8. Treatment of neutrophils with anti-CD14 antibodies and treatment of serum with anti-LPS binding protein (LBP) antibodies resulted in the inhibition of priming for respiratory burst by Pam3CSK4. It should be noted that LBP could not replicate the effects of serum in priming of neutrophils for respiratory burst by Pam3CSK4. Serum LBP bound to immobilized Pam3CSK4. Pam3CSK4 induced the interleukin-8 (IL-8) production by leukocytes in a serum-dependent manner. Further, Pam3CSK4-induced priming of neutrophils for respiratory burst was not inhibited by the LPS antagonists LA-14-PP, Rhodobacter sphaeroides LPS, or E5531, and Pam3CSK4-induced IL-8 production by leukocytes was not affected by LPS antagonist, E5531, indicating that Pam3CSK4 was recognized by a different receptor than LPS. Thus, Pam3CSK4 and LPS had similar biological activities and similar requirement to act on leukocytes, but were recognized by different receptors. Serum in the action of Pam3CSK4 on leukocytes was not replicated by LBP, suggesting that Pam3CSK4 might be disaggregated by serum to result in the activation of leukocytes.