The inhibition of the aggregation in protein solutions is currently a subject of great interest in many research fields, from the study of protein-misfolding related diseases to pharmaceutics, biotechnology, and food science. alpha(sl)-Casein, one of the four types of caseins, which are the largest protein component of bovine milk, has been found to hinder the aggregation process of several proteins, including the amyloid beta-peptide, involved in Alzheimer's disease. To shed light into the mechanisms by which casein exerts this chaperon-like protective action, we studied its effect on the different steps of the aggregation process of concanavalin A, by means of both static and dynamic light scattering, thioflavin T and ANS fluorescence, circular dichroism, and atomic force microscopy. Our results show that casein has a poor effect on the first step of the process leading to the formation of amyloid-like structures. On the contrary, it has a marked effect on the second step of the process, ascribable to clusters condensation and compaction, up to the formation of very large aggregates. Such an effect requires a molar ratio of casein larger than that necessary to inhibit the fibrillogenesis of the amyloid beta-peptide, thus, suggesting a different mechanism of interaction of casein, depending on both conformational properties and relative size of the aggregating molecules.