Fibrillar deposits of aggregated amyloid β peptide (Aβ) comprise the characteristic senile plaques in Alzheimer's disease (AD). The plaques, which are found in the brain parenchyma, are composed of both 40- and 42-residue Aβ. It has long been thought that these fibrillar deposits were responsible for the neuronal loss in AD, but more recent studies have supported the idea that soluble fibril precursor species may play a more crucial role in AD pathogenesis. We used human THP-1 monocytes as a model system to determine what Aβ aggregation species induces the strongest proinflammatory response. These cells are sensitive to Aβ and will secrete tumor necrosis factor α (TNFα) in response. We found that the highest levels of Aβ(1-42)-induced TNFα production occurred 48 hours after the peptide had been reconstituted in water and incubated at 4°C. Longer incubation of the peptide decreased its ability to stimulate the cells and more concentrated Aβ(1-42) solutions were unable to stimulate the cells at any time point. Atomic force microscopy imaging of the Aβ(1-42) aggregation solutions showed predominantly globular species after reconstitution progressing to mixed population of globules and small rodlike species at 24-72 hours and finally long fibrillar species upon further aging at 4°C. The more concentrated, faster-aggregating solutions showed increased fibrillar content within 24 hours. Aβ(1-42) aggregation solutions were also monitored by thioflavin T (ThT) fluorescence. Fluorescence correlated positively with increased aggregation but did not correlate with cellular stimulation. The findings suggest that an intermediate Aβ aggregation species is the most biologically active form.