Background: The use of whole blood (WB) in studying lipopolysaccharide (LPS)‐induced cellular activation preserves the milieu in which LPS‐cell interaction occurs in vivo. However, little information is available on using such a system at a single‐cell level. We evaluated LPS binding and cell activation in WB by using flow cytometry. The influence of heparin or EDTA as anticoagulants was also addressed. Methods: Blood was obtained from healthy donors in EDTA and/or heparin tubes. Biotinylated LPS (LPSb) was used to evaluate cell binding of LPS in WB. Cells were surface stained with appropriate antibodies and LPSb was detected by adding streptavidin‐allophycocyanin (APC). LPS‐induced activation was evaluated by the expression of surface activation markers and by the detection of intracellular tumor necrosis factor‐alpha (TNF‐α). Results: LPSb bound promptly to monocytes in EDTA‐ and heparin‐treated blood. In EDTA‐treated blood, membrane‐bound LPSb decreased after 60 min of incubation, whereas it remained detectable in heparinized blood during the 6 h of incubation. LPS induced TNF‐α and enhanced the expression of HLA‐DR in monocytes, as well as the expression of CD69 in T and B lymphocytes. Induction of both TNF‐α in monocytes and CD69 in lymphocytes was more efficient in heparinized blood. Conclusion: Detection of membrane‐bound LPSb on monocytes differed in EDTA or heparin‐treated blood, and cell activation was better obtained in heparinized blood. Cytometry (Clin. Cytometry) 50:14–18, 2002. © 2002 Wiley‐Liss, Inc.