We used biotinylated LPS (LPSb) and flow cytometry to study LPS-monocyte interaction and LPS-induced cellular activation in whole blood from septic patients (SP). Expression of surface activation markers was evaluated on monocytes (HLA-DR) and T lymphocytes (CD69 and CD95), and intracellular TNF- alpha on monocytes. Saturating curve and kinetics of LPSb detection on monocytes were similar in SP and healthy volunteers (HV). LPSb bound to monocytes was detected after 5 min of incubation in both groups, with a more pronounced decay in SP. Monocytes from SP had a lower expression of HLA-DR as compared to HV, both constitutive and upon LPS stimulation. The proportion of monocytes producing TNF- alpha after LPS stimulus was higher in HV than SP (mean plus or minus SD = 25.2 plus or minus 14.2% and 2.2 plus or minus 2.6%, respectively, P < 0.001). LPS-induced CD69 on T CD8 super(+) and CD8 super(-) lymphocytes was similar for patients and controls. Expression of CD95 on T lymphocytes was higher in SP as compared to HV on T CD8 super(+) cells (GMFI, mean plus or minus SD = 22.3 plus or minus 14.6 and 8.6 plus or minus 5.0, respectively, P = 0.01) and CD8 super(-) cells (GMFI, mean plus or minus SD = 28.3 plus or minus 7.7 and 14 plus or minus 4.3 respectively, P < 0.001). Thus, monocytes and lymphocytes seem to respond differently to LPS in septic patients. Monocyte hyporesponsiveness appears not to be related to a decreased binding capacity of LPS, but rather to an impaired signal transduction.