The interaction of nanoparticles (Nps) with body fluids may induce conformational changes in the proteins present in the medium. Such interactions could induce functional loss or important modifications in some proteins, and trigger cellular events induced by the Np-protein moiety. As metal oxide nanoparticles are widely used for various applications, the interaction of four different metal oxide Nps (ZnO, TiO2, CeO2 and Al2O3) with three of the main protein fractions from human plasma (albumin, fibrinogen and globulins) was characterized by fluorescence and Fourier-transform infrared (FTIR) spectroscopy. The pattern of Np-protein interaction was shown to vary depending on the type of Np. For ZnO Nps, a strong interaction was observed, which induced a decrease in the thermal stability of both fibrinogen and albumin at a low temperature, interfering with the clotting activity of fibrinogen. TiO2 and CeO2 Nps showed lower effects, while for Al2O3 Nps only a slight or null interaction was observed at physiological pH. Moreover, the influence of pH was characterized for albumin, showing that the Np-protein interaction has an important dependence on the Np surface charge. The conformational changes induced by metal oxide Nps in the secondary structure of albumin are principally the transformation of α-helices into β-sheet structures. The interaction, with the exception of Al2O3 nanoparticles at basic pH, could take place in the domain II of the protein, formed mainly by hydrophobic and positive residues.