Two trace impurities in the bulk drug lisinopril were detected by means of high-performance liquid chromatography coupled with mass spectrometry (HPLC/MS) with a simple and sensitive method suitable for HPLC/MSn analysis.The frag-mentation behavior of lisinopril and the impurities was investigated,and two unknown impurities were elucidated as 2-(6-amino-1-(1-earboxyethylamino)-1-oxohexan-2-ylamino)-4-phenylbutanoic acid and 6-amino-2-(1-carboxy-3-phenylpro-pylamino)-hexanoic acid on the basis of the multi-stage mass spectrometry and exact mass evidence.The proposed structures of the two unknown impurities were further confirmed by nuclear magnetic resonance (NMR) experiments after preparative isola-tion.