As the population ages, the need to develop methods to understand and intercept the processes responsible for protein aggregation diseases is becoming more urgent. The aggregation of the protein beta-amyloid (A beta) has been implicated in Alzheimer's Disease (AD); however, whether the toxic species is a large, insoluble aggregate or some lower order form is not yet known. Agents that can modulate the aggregation state of A beta could resolve this controversy by facilitating our understanding of the consequences of aggregation and its underlying mechanism. To date, however, ligands that bind to specific forms of A beta have not been identified. To address this deficiency, we tested whether phage display could yield such ligands by screening libraries against A beta in two different states: monomeric or highly aggregated. Intriguingly, the peptides selected had different effects on A beta aggregation. Peptides selected for binding to monomeric A beta did not perturb aggregation, but those selected using highly aggregated A beta increase the rate of aggregation drastically. The latter also alter the morphology of the resulting aggregate. The ability of a peptide to promote aggregation correlated with its affinity for the N-terminal 10 residues of A beta. This result indicates that the mechanism by which the peptides influence aggregation is related to their affinity for the A beta N-terminus. Thus, the identification of compounds that bind to this A beta section can afford agents that affect aggregation. Moreover, the data suggest that endogenous ligands that interact with the N-terminal region can influence the propensity of A beta to form aggregates and the morphology of those that form. Our data highlight the utility of phage display for identifying ligands that bind to target proteins in different states, and they indicate that such agents can be used to perturb protein aggregation.