The growth inhibition and the induction of apoptosis brought about by soyasaponins extracted from soy flour ( Glycine max (L.)) and concentrated for soyasapogenols A and B formed by hydrolysis were tested for cytoactivity in the human hepatocellular carcinoma cell line Hep-G2. Concentrated soyasapogenol A (SG-A) and soyasapogenol B (SG-B) extracts contained approximately 69.3% and 46.2% of their respective aglycones (soyasapogenols) assessed by HPLC and ESI-MS, while the soyasaponin extract (TS), derived from crude methanol extraction, did not contain any detectable amounts of SG-A or SG-B. An MTT viability assay showed that all three extracts had an effect on Hep-G2 proliferation in a dose-response manner with 72 h LC50 values of 0.594+/-0.021 mg/mL for TS, 0.052+/-0.011 mg/mL for SG-A, and 0.128+/-0.005 mg/mL for SG-B. Apoptotic cells were determined by flow cytometry cell cycle analysis and confocal laser scanning microscopy (CLSM). Cell cycle analysis indicated a significant ( P< 0.05) greater sub-G1 buildup of apoptotic cells at 24 h (25.63+/-2.1%) and 72 h (47.1+/-3.5%) for the SG-A extract compared to SG-B, whereas the TS extract produced only a minor buildup of sub-G1 cells. CLSM confirmed a morphological change of all treatments after 24 h, at the respective LC50 concentrations. These results show that the samples that contained mainly soyasapogenols A and B showed a greater ability to inhibit proliferation of cultured Hep-G2 when compared to a total soyasaponin extract that did not contain any soyasapogenols.