The 42-aa-long β-amyloid protein—Aβ_1-42—is thought to play a central role in the pathogenesis of Alzheimer's disease (AD) (Walsh and Selkoe, 2007). Data from AD brain (Shankar et al., 2008), transgenic APP (amyloid precursor protein)-overexpressing mice (Lesné et al., 2006), and neuronal cultures treated with synthetic Aβ peptides (Lambert et al., 1998) indicate that self-association of Aβ_1-42monomers into soluble oligomers is required for neurotoxicity. The function of monomeric Aβ_1-42is unknown. The evidence that Aβ_1-42is present in the brain and CSF of normal individuals suggests that the peptide is physiologically active (Shoji, 2002). Here we show that synthetic Aβ_1-42monomers support the survival of developing neurons under conditions of trophic deprivation and protect mature neurons against excitotoxic death, a process that contributes to the overall neurodegeneration associated with AD. The neuroprotective action of Aβ_1-42monomers was mediated by the activation of the PI-3-K (phosphatidylinositol-3-kinase) pathway, and involved the stimulation of IGF-1 (insulin-like growth factor-1) receptors and/or other receptors of the insulin superfamily. Interestingly, monomers of Aβ_1-42carrying the Arctic mutation (E22G) associated with familiar AD (Nilsberth et al., 2001) were not neuroprotective. We suggest that pathological aggregation of Aβ_1-42may also cause neurodegeneration by depriving neurons of the protective activity of Aβ_1-42monomers. This “loss-of-function” hypothesis of neuronal death should be taken into consideration when designing therapies aimed at reducing Aβ burden.