Abstract: OBJECTIVE To establish an efficient and simple HPLC-MS/MS method for determination of flucloxacillin in newborn plasma, and to investigate the interaction between ambroxol and flucloxacillin in newborns. METHODS The samples were analyzed by API4000 HPLC-MS/MS. Ultimate XB-C₁₈ column(2.1 mm×100 mm, 5 μm) were carried out. The mobile phase was composed of water-0.1% formic acid(A) and acetonitrile-0.1% formic acid(B). The quantitative analysis of the ion transitions were monitored at m/z 452.6→284.2 for flucloxacillin and m/z 821.4→397.3 for rifampicin(internal standard). RESULTS The linear range of flucloxacillin under this analysis method was 0.20-80 ng·mL^-1, and the lower limit of quantification was 0.20 ng·mL^-1; The intra-day and inter-day precision of flucloxacillin were both less than 8.23%; The extraction recovery was in the range of 85.3%-89.2%, and the matrix effect was between 89.3%-92.3%; The stability of plasma samples was good under conditions of 12 h at room temperature, 4 h at room temperature after treatment, repeated freeze-thaw for 3 times, and -20 ℃ freezing for 30 d. The results of clinical samples indicated that the combination of ambroxol could significantly increase the blood concentration of flucloxacillin. CONCLUSION The established HPLC-MS/MS method is accurate, sensitive and can be used for the determination of flucloxacillin concentration in neonatal plasma. The results of clinical samples indicate that ambroxol can significantly increase the blood concentration of flucloxacillin. There are drug interactions between ambroxol and flucloxacillin.