Toll-like receptors (TLRs) are involved in the recognition of bacterial products and thus participate in the induction of the inflammatory cascade. However, much less is known about the evolution of leucocyte TLR expression during human inflammatory stress. We hypothesized that a decrease in leucocyte TLRs could account for the so-called tolerance or hyporesponsiveness state to subsequent stimulation with bacteria-derived products. Because of the profound monocytopenia that ensues after in vivo lipopolysaccharide (LPS) challenge, we also compared monocyte TLR expression using two different techniques of flow cytometric gating. In a first set of experiments, 17 healthy volunteers underwent LPS challenge. Blood was drawn at different time-points and analysed by flow cytometry using light scatter gating and one-colour analysis to assess the expression of the tumour necrosis factor receptor (TNFR) and TLR2 and TLR4 on both monocytes and granulocytes. In a second set of experiments, the assessment of those receptors was made using a more specific gating method that utilized light scatter and CD14 immunofluorescence in a two-colour analysis. This was performed using whole blood drawn from five healthy volunteers and incubated ex vivo for different time periods with or without LPS and in 12 volunteers who underwent LPS challenge in vivo. The pattern of expression for monocyte TNFR was similar for both types of gating. Using only the light scatter gating, an initial drop of TLR 2 and 4 was observed on monocytes. By contrast, when using light scatter x immunofluorescence gating, an up-regulation of these two receptors following both in vivo and in vitro LPS exposure was observed. LPS up-regulates the expression of TLRs on monocytes and granulocytes. Depending upon the methodology utilized, contrasting results were obtained with respect to TLR2 and TLR4 expression. The flow cytometric gating technique used is of importance in determining cellular TLR2 and TLR4 expression, especially in blood samples exhibiting significant monocytopenia.