Magnetic nanoparticles (MNPs) have great potential for a wide use in various biomedical applications due to their unusual properties. It is critical for many applications that the biological effects of nanoparticles are studied in depth. To date, many disparate results can be found in the literature regarding nanoparticle-biological factors interactions. This review highlights recent developments in this field with particular focuses on in vitro MNPs-cell interactions. The effect of MNPs properties on cellular uptake and cytotoxicity evaluation of MNPs were discussed. Some employed methods are also included. Moreover, nanoparticle-cell interactions are mediated by the presence of proteins absorbed from biological fluids on the nanoparticle. Many questions remain on the effect of nanoparticle surface (in addition to nanoparticle size) on protein adsorption. We review papers related to this point too. The current development of magnetic nanoparticles (MNPs) requires a better understanding of its biological effects. A great deal of work has been conducted to study MNPsbiological factors interactions. Many researchers bend themselves to in vitro cell-based assessment. In this work, the current knowledge of how the in vitro cultured cells can interact with the exposed colloid MNPs is discussed as well as the effect of nanoparticle size and surface properties on cell responses. The specific cell line selected for in vitro assay is intended to model a response or phenomenon likely observed or sensitized by particles in vivo. Here, the frequent lack of consistency or predictability between in vitro models and in vivo observations, which is because of the different biological conditions particles exposed to and the changed cell phenotype against primary cell types, is out of our consideration. As we have known, cell monocultures as measured by in vitro assays rarely react in such isolated pathways in native tissues comprosed of multiple, dynamically communicative cell types that produce non-linear and correlated response to foreign materials.Adsorption of proteins onto the nanoparticle surface happens immediately after particles come in contact with a biological fluid, and it is the proteins associate with nanoparticles, leading to a protein "corona" which defines the biological identity of the particle. Here, we also try to review some current work on associated protein "corona" when nanoparticles were incubated in biological medium.