Novel surface active maghemite nanoparticles (SAMNs) possessing peculiar colloidal properties and surface characteristics are able to covalently bind biomolecules. The interactions of SAMNs and rhodamine derivatized SAMNs (SAMN@RITC) with proteins from cell culture medium were studied by gel electrophoresis and mass spectrometry. Among the 3000 proteins present in fetal calf serum, SAMNs and SAMN@RITC give rise to the formation of a self-assembled corona shell with 22 selected proteins, representing minor plasma proteins, among which α-2-HS- glycoprotein stands out. Bovine serum albumin (BSA), representing 80% of the total serum proteins, shows negligible absorption on the SAMN surface. Nevertheless, SAMNs are able to bind BSA, upon incubation in pure BSA solutions. The interaction between SAMNs and BSA was investigated by optical spectroscopy, circular dichroism, Fourier transform infrared spectroscopy, and transmission electron microscopy. BSA binding resulted a time-consuming process, nevertheless experimental results showed the interaction of 6 ± 2 BSA molecules per nanoparticle, and optical spectra indicate remarkable changes in SAMN optical features, as well as circular dichroism proved secondary structure alteration of bound BSA, suggesting that the protein needs to adapt its structure to adhere to nanoparticle surface. The selectively bound protein corona shell, formed upon SAMNs incubation in calf serum, was responsible for the characteristic behavior when SAMNs were tested for cell internalization and cytotoxicity on HeLa cells. Cytotoxicity of SAMN preparations was extensively studied, and was negligible up to 100 μg mL -1. Moreover, nanoparticle uptake proceeded for long times, suggesting a correlation between internalization and stability of covalently bound self-assembled protein corona, representing a unique example of magnetic nanoparticle opsonization via covalent binding. We suggest that SAMN based nanobiocomposites can be employed for the preparation of self-assembled opsonized nanoparticles as future candidates for biomedical applications.