Protective effects of Transthyretin (TTR) ameliorate Alzheimer's pathology in mice models. This suggests a possible route for cellular defense mechanism against toxic effects of protein misfolding and aggregation. This view is further supported by the results from in vitro experiments showing direct interactions between TTR and amyloid-β (Aβ) peptides. However, the molecular mechanism of the TTR-Aβ interactions is currently unknown primarily due to heterogeneities associated with self-association of both TTR and Aβ. Here we investigate the mechanism of TTR-Aβ interactions by examining the effects of WT-human, a monomeric mutant and murine TTR on the kinetics of aggregation of both Aβ1-40 and Aβ1-42. The three of forms of TTR differ in terms of stability as tetramers or monomers, e.g., the WT-human and murine TTR are primarily tetramers but the tetramers of murine TTR are more stable. To monitor the aggregation of Aβ but not of TTR we use a recently developed aggregation assay based on the fluorescence quenching of tetramethyl rhodamine (TMR)-labeled Aβ. Our data indicate that all three TTR variants delay aggregation of both Aβ1-40 and Aβ1-42. However, the effects are strongest for the monomeric mutant and weakest for the murine TTR. Kinetic measurements of fluorescence resonance energy transfer between native tryptophan residues of TTR and the EDANS-labeled Aβ indicate that binding of TTR is dependent on the aggregation status of Aβ with maximal binding occurring to larger aggregates of Aβ. Additionally, the modulatory effects of TTR are stronger on Aβ1-42 than on Aβ1-40 consistent with the known higher oligomerization propensity for Aβ1-42. Taken together our results indicate that the tetramers of WT-human TTR dissociates to monomers to bind to the aggregation intermediates of Aβ to subsequently slow down the growth of these intermediates.