Fibrillogenesis of the amyloid beta-protein (Abeta) is a seminal pathogenetic event in Alzheimer's disease. Inhibiting fibrillogenesis is thus one approach toward disease therapy. Rational design of fibrillogenesis inhibitors requires elucidation of the stages and kinetics of Abeta fibrillogenesis. We report results of studies designed to examine the initial stages of Abeta oligomerization. Size exclusion chromatography, quasielastic light scattering spectroscopy, and electron microscopy were used to characterize fibrillogenesis intermediates. After dissolution in 0.1 M Tris-HCl, pH 7.4, and removal of pre-existent seeds, Abeta chromatographed almost exclusively as a single peak. The molecules composing the peak had average hydrodynamic radii of 1.8 +/- 0.2 nm, consistent with the predicted size of dimeric Abeta. Over time, an additional peak, with a molecular weight >100,000, appeared. This peak contained predominantly curved fibrils, 6-8 nm in diameter and <200 nm in length, which we have termed "protofibrils." The kinetics of protofibril formation and disappearance are consistent with protofibrils being intermediates in the evolution of amyloid fibers. Protofibrils appeared during the polymerization of Abeta-(1-40), Abeta-(1-42), and Abeta-(1-40)-Gln22, peptides associated with both sporadic and inherited forms of Alzheimer's disease, suggesting that protofibril formation may be a general phenomenon in Abeta fibrillogenesis. If so, protofibrils could be attractive targets for fibrillogenesis inhibitors.