A rapid, sensitive and selective high-performance liquid chromatography-tandem mass spectrometric method (HPLC-MS-MS) has been developed and validated for the determination of soyasaponins Ba and Bb in human serum using glycyrrhizin as internal standard (I.S.). Soyasaponins Ba and Bb were extracted from human serum by liquid-liquid extraction and cleaned up by C(18) solid-phase extraction (SPE), followed by separation on a C(18) reversed-phase column using acetonitrile/water containing 0.025% acetic acid as a mobile phase for gradient elution. Soyasaponins Ba and Bb, and I.S. were ionized by negative ion pneumatically assisted electrospray and detected by HPLC-MS-MS in the multiple-reaction monitoring (MRM) mode using precursor-->product ion combinations at m/z 958-->940, 942-->924 and 822-->351, respectively. The calibration curves were linear (r(2)>0.991) in the concentration range of 0.5-100.0 ng/mL, with lower limits of quantification of 0.5 and 0.3 ng/mL for soyasaponins Ba and Bb, respectively, in human serum. Intra-day and inter-day relative standard deviations (R.S.D.) were less than 7.9 and 11.3%, respectively. The mean recoveries of soyasaponins Ba and Bb ranged from 92 to 101% and from 85 to 94%, respectively.