beta A4 peptide (beta AP) accumulates in amyloid plaques of Alzheimer's disease and may contribute to neuronal degeneration. Conflicting observations have been reported regarding the direct in vitro and in vivo neurotoxicity of beta AP. We have assessed in vitro beta AP toxicity in high density primary rat hippocampal cultures and found marked lot-to-lot differences in the neurotoxic properties of beta AP. One lot of beta AP from a commercial supplier resulted in significant direct neurotoxicity at 10 microM, while 2 other lots from the same supplier were essentially nontoxic. Three additional lots of beta AP from unrelated sources were also nontoxic at 10 microM. Initial biochemical characterization has not yet revealed any marked differences among the various lots of beta AP. Low levels of endotoxin (ca., 1 EU/ml) were detected in several beta AP preparations but did not correlate with neurotoxicity. Our observation that lot-to-lot variability of beta AP occurred even under identical in vitro culture conditions may account for part of the present controversy in this area.