This study demonstrates that lipopolysaccharide (LPS) mediates induction of transcription factor NF kappa B and activation of the cytomegalovirus (CMV) promoter-enhancer in the SW480 cell line. These cells do not express a functional membrane CD14. The LPS response in SW480 cells was weaker and markedly slower than the tumor necrosis factor (TNF) response. Pretreatment with TNF for 72 h inhibited both TNF, tumor necrosis factor receptor (TNFR) p55, TNFR p75, and LPS-mediated activation of nuclear factor -kappa B (NF kappa B), whereas pretreatment with LPS only inhibited the LPS response. TNFR p55 antibody pretreatment resulted in marked inhibition of the LPS response, while pretreatment with TNFR p75 antiserum only had a weak inhibitory effect. Flowcytometric analysis showed that LPS binding as well as expression of TNFR p55 and TNFR p75 were not affected by LPS or TNF pretreatment, indicating that the observed inhibition is not due to reduction of specific binding sites at the cell surface. The results suggest that LPS signaling in SW480 cells involves intracellular components which may be depleted or inactivated via TNFR p55, indicating that the LPS and TNFR p55 pathways overlap. We propose that TNFR p55 can mediate activation of NF kappa B and cytomegalovirus promoter-enhancer in SW480 cells via two distinct mechanisms, one which is activated only via TNFR p55 and leads to rapid activation of NF kappa B, and another which is overlapping with the LPS pathway.