Nanoparticles interfere with protein amyloid formation. Catalysis of the process may occur due to increased local protein concentration and nucleation on the nanoparticle surface, whereas tight binding or a large particle/protein surface area may lead to inhibition of protein aggregation. Here we show a clear correlation between the intrinsic protein stability and the nanoparticle effect on the aggregation rate. The results were reached for a series of five mutants of single-chain monellin differing in intrinsic stability toward denaturation, for which a correlation between protein stability and aggregation propensity has been previously documented by Szczepankiewicz et al. [Mol. Biosyst.20107 (2), 521-532]. The aggregation process was monitored by thioflavin T fluorescence in the absence and presence of copolymeric nanoparticles with different hydrophobic characters. For mutants with a high intrinsic stability and low intrinsic aggregation rate, we find that amyloid fibril formation is accelerated by nanoparticles. For mutants with a low intrinsic stability and high intrinsic aggregation rate, we find the opposite--a retardation of amyloid fibril formation by nanoparticles. Moreover, both catalytic and inhibitory effects are most pronounced with the least hydrophobic nanoparticles, which have a larger surface accessibility of hydrogen-bonding groups in the polymer backbone.