The monoclonal antibody 45 reacts with Polymyxin B. The antibody binds to free Polymyxin B as well as to Polymyxin B already bound to LPS.The peptide antibiotic Polymyxin B (PMB) binds to bacterial endotoxin (lipopolysaccharide, LPS). The interaction of PMB with LPS involves ionic forces between amino groups in PMB and negatively charged phosphate and carboxyl groups in the lipid A-Kdo region. PMB has relevance for endotoxin research in at least two ways: first, PMB reacts with LPS of many species regardless of varied serospecificity, and thus it can be used as a general probe for measuring or detecting LPS or lipid A. Second, binding of PMB to LPS may result in neutralization of the detrimental effects of LPS either in vitro or in vivo. Monoclonal antibody 45 enables the possibilities to study quantitatively the interaction of PMB and LPS.