A highly sensitive ELISA for the determination of polymyxin B sulfate (PMB) was developed which is capable of measuring as low as 32pg/ml. Anti-PMB antibody was obtained by immunizing rabbits wiht PMB conjugated with mercaptosuccinyl bovine serum albumin (MS. BSA) using N-(γ-maleimidobutyryloxy) succinimide (GMBS) as a heterobifunctional coupling agent. An enzyme marker was similarly prepared by coupling PMB with horseradish peroxidase (HRP) employing GMBS. This ELISA showed very low reactivity with the PMB analogue, polymyxin E (0.05%). The values for PMB concentration detected by this assay were comparable with those detected by the bioassay. Moreover, the ELISA was about 10000 times more sensitive in detecting PMB at lower concentrations. Serum PMB concentration after the oral administration of a PMB tablet to human subjects was determined by the ELISA. PMB was rapidly absorbed from the gastrointestinal tract after the administration. then slowly decreased. These results indicate that the ELISA may be a valuable tool for studies of the pharmacokinetics and pharmacodynamics of the anti-endotoxin drug, PMB.