Lipid second messengers are gaining recognition as important mediators of extracellular signals. One such lipid, ceramide, generated from membrane sphingomyelin following stimulation with TNF-alpha, IL-1 beta, or IFN-gamma, activates ceramide-activated kinase (CAK). A recent study demonstrated that LPS activated CAK without generating ceramide, suggesting that the LPS stimulation of cells mimics the second messenger function of ceramide. To compare ceramide to LPS signaling, we assessed the ability of LPS-responsive (Lpsn) and LPS-hyporesponsive (Lpsd) macrophages to respond directly to ceramide for enhanced expression of LPS-inducible genes. In contrast to macrophages from C3H/Ouj (Lpsn) mice, C3H/Hej (Lpsd) macrophages failed to respond to cellpermeable analogues of ceramide (C2,C6,C16) or sphingomyelinase. These results suggest that a common critical molecule, encoded by the Lps gene, regulates both ceramide and LPS signaling pathways.