Recent data suggest that soluble, non-fibrillar assemblies of the amyloid β-protein (Aβ) may mediate the synaptic deficits that characterize the early stages of Alzheimer's disease. Consequently, much effort has been expended in isolating and studying a variety of different Aβ assemblies. Here, we describe the use of immunoprecipitation/western blotting and size exclusion chromatography/western blotting to characterize Aβ present in conditioned medium from cultured cells, human cerebrospinal fluid, and human cortex extracted with aqueous buffer, detergent, and formic acid.