antibacterial activities against gram-positive and negative aerobic and anaerobic bacteria and is widely used in clinics. PAPM is used for treating severe infectious diseases, such as complicated urinary tract infections, meningitis and septicemia in pediatrics, and also in extremely low birth weight infants. However, its safety for neonates has not yet been established, because no clinical trial in neonates which focus on the pharmacokinetics of PAPM was performed. In this study, we attempted to determine PAPM in neonate plasma after an intravenous dose of PAPM in order to establish the correct dasage regimen of PAPM. However, it is quite difficult to quantify the sub-microgram order of plasma PAPM using a conventional reversed-phase high-performance liquid-chromatography (HPLC) method,1 due to the small sample volume (20 – 30 μL as plasma) available from neonatal or immature infants. As PAPM appeared as two diastereomer peaks in the previous method; it is also a fetal drawback in view of the assay sensitivity. We established a microanalytical method using ion pair HPLC without separating the distereomeric isomer.