Magnetic gold nanoparticles are used in various biomedical, biochemistry, and biotechnology applications due to their controllable size distribution, long-term stability, reduced toxicity, and biocompatibility. Different coating materials, such as proteins, carbohydrates, lipids, and polyphenols, are applied to enhance the biocompatibility of nanoparticles. In this study, the effects of surface coatings of core-shell structured Fe@Au nanosphere magnetic nanoparticles with regard to antioxidant capacity and cytotoxic, anticarcinogenic, and genotoxic properties were investigated. The obtained results demonstrated that avidin-coated Fe@Au nanospheres had higher antioxidant capacities than uncoated nanospheres. Neither avidin-coated nor uncoated nanoparticles had a cytotoxic effect on normal cells (human gingival fibroblast cell line, HGF-1). In addition, they had anticarcinogenic effects on human cervical carcinoma (HeLa), human breast adenocarcinoma (MCF-7), and human colorectal adenocarcinoma (CCL-221). The genotoxic effects of nanoparticles were also evaluated with DNA tail damage ratio.