The peptide Polymyxin B (PMB) is a cyclic amphipathic peptide antibiotic that binds to endotoxins (lipopolysaccharide, LPS). Binding of PMB to LPS results in neutralization of LPS of gram-negative bacteria. For the study of PMB-LPS interactions PMB conjugates proved to be superior to the application of anti-PMB antibodies. PMB conjugates are useful general probes to detect or measure lipid A and LPS of various species using simple methods. These conjugates are also useful for staining of bacteria. The conjugates obviate the need for many specific antisera. Furthermore PMB conjugates are useful tools in investigations of LPS binding protein (LBP) and bactericidal permeability increasing protein (BPI) interactions with LPS. In conclusion, PMB conjugates are useful probes for endotoxin research.