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. Antibodies to PMB allow for the quantitative study of interaction of PMB/LPS interactions.