Standardized extract from the leaves of the Ginkgo biloba tree, labeled EGb761, has been used in clinical trials for its beneficial effects on brain functions, particularly in connection with agerelated dementias and Alzheimer's disease (AD). Substantial experimental evidence indicates that EGb761 protects against neuronal damage from a variety of insults, but its cellular and molecular mechanisms remain unknown. Using a neuroblastoma cell line stably expressing an AD-associated double mutation, we report that EGb761 inhibits formation of amyloid-␤ (A␤) fibrils, which are the diagnostic, and possibly causative, feature of AD. The decreased A␤ fibrillogenesis in the presence of EGb761 was observed both in the conditioned medium of this A␤-secreting cell line and in solution in vitro. In the cells, EGb761 significantly attenuated mitochondrion-initiated apoptosis and decreased the activity of caspase 3, a key enzyme in the apoptosis cell-signaling cascade. These results suggest that (i) neuronal damage in AD might be due to two factors: a direct A␤ toxicity and the apoptosis initiated by the mitochondria; and (ii) multiple cellular and molecular neuroprotective mechanisms, including attenuation of apoptosis and direct inhibition of A␤ aggregation, underlie the neuroprotective effects of EGb761.