Activation of the classical complement pathway has been proposed as a mechanism of neurodegeneration in Alzheimer's disease. This activation is a result of the binding of C1q to amyloid beta-peptide (A beta). Recent work has shown that A beta/C1q binding has an additional consequence: enhanced formation of the neurotoxic, fibrillar, cross beta-pleated A beta configuration. Here we show that C1q enhances A beta aggregation at physiologically relevant, nanomolar concentrations of the peptides, and demonstrate that the kinetics of this enhancement are consistent with a nucleating interaction. We also show that the intact, multimeric structure of C1q, which offers multiple A beta binding sites spaced at 2-3 nm intervals, is required.