Extracellular amyloid β peptides (Aβs) have long been thought to be a primary cause of Alzheimer's disease (AD). Now, detection of intracellular neuronal Aβ_1–42 accumulation before extracellular Aβ deposits questions the relevance of intracellular peptides in AD. In the present study, we directly address whether intracellular Aβ is toxic to human neurons. Microinjections of Aβ_1–42 peptide or a cDNA-expressing cytosolic Aβ_1–42 rapidly induces cell death of primary human neurons. In contrast, Aβ_1–40, Aβ_40–1, or Aβ_42–1 peptides, and cDNAs expressing cytosolic Aβ_1–40 or secreted Aβ_1–42 and Aβ_1–40, are not toxic. As little as a 1-pM concentration or 1500 molecules/cell of Aβ_1–42 peptides is neurotoxic. The nonfibrillized and fibrillized Aβ_1–42 peptides are equally toxic. In contrast, Aβ_1–42 peptides are not toxic to human primary astrocytes, neuronal, and nonneuronal cell lines. Inhibition of de novo protein synthesis protects against Aβ_1–42 toxicity, indicating that programmed cell death is involved. Bcl-2, Bax-neutralizing antibodies, cDNA expression of a p53^R273H dominant negative mutant, and caspase inhibitors prevent Aβ_1–42-mediated human neuronal cell death. Taken together, our data directly demonstrate that intracellular Aβ_1–42 is selectively cytotoxic to human neurons through the p53–Bax cell death pathway.