The mechanism of a novel assay technique for nanoparticle-enhanced immunoturbidimetric assays is investigated. In a mixture of latex particles of two different sizes, coated with antibodies of different affinities, the aggregation behavior is monitored, which correlates with the antigen concentration. At low antigen concentrations, only the bigger latex particles coated with the high-reactivity antibody aggregate, whereas at higher antigen concentrations, the smaller latices coated with a lower reactivity antibody follow up in the aggregation process. This is shown for an immunoassay (C-reactive protein) by theoretical considerations based on a diffusion-controlled reaction and by transmission electron microscopy, analytical ultracentrifugation, and static light scattering as complementary qualitative and quantitative analytical techniques.