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Abstract: Eubacterium limosum ZL-II is an anaerobic bacterium with demethylated activity, which was isolated from human intestinal bacteria in our previous work. In this study, the flavonolignan constituents of Silybi Fructus were biotransformed by E. limosum 1 [1] Eubacterium limosum. ZL-II, producing four new transformation products — demethylisosilybin B (T1), demethylisosilybin A (T2), demethylsilybin B (T3) and demethylsilybin A (T4), among which T1 and T2 were new compounds. Their chemical structures were identified by ESI-TOF/MS, 1H NMR, 13C NMR, HMBC and CD spectroscopic data. The bioassay results showed that the transformation products T1–T4 exhibited significant inhibitory activities on Alzheimer's amyloid-? 42 (A?42 2 [2] Amyloid-?. ) aggregation with IC50 values at 7.49?M–10.46?M, which were comparable with that of the positive control (epigallocatechin gallate, EGCG 3 , [3] Epigallocatechin gallate. at 9.01?M) and much lower than those of their parent compounds (at not less than 145.10?M). The method of biotransformation by E. limosum ZL-II explored a way to develop the new and active lead compounds in Alzheimer's disease from Silybi Fructus. However, the transformation products T1–T4 exhibited decreased inhibitory activities against human tumor cell lines comparing with their parent compounds. [Copyright &y& Elsevier]