It is reported on the binding of lipopolysaccharide (LPS), a virulence factor of Gram-negative bacteria, with important cells of the human immune system, macrophages, applying isothermal titration calorimetry (ITC). Macrophages are responsible for the secretion of a variety of cytokines such as interleukins and tumor-necrosis-factor-alpha (TNFalpha) after stimulation with LPS, which may lead at sufficient LPS concentrations to the highly dangerous sepsis syndrom. The production of cytokines can be blocked by the addition of suitable antibiotics such as polymyxin B (PMB). The ITC measurements show that the interaction of LPS to macrophages is connected with a strong exothermic reaction probably in first line due to the binding of LPS to cell membrane proteins such as CD14 and membrane-bound mLBP, which are well-known LPS binding structures. This exothermic reaction can be considerably reduced by the addition of PMB to the cells, showing that the reduced cellular activation is connected with a strongly reduced interaction of LPS with the membrane surface. These first data prove the applicability of ITC for the LPS:cell interaction. (C) 2003 Elsevier B.V. All rights reserved.