Superparamagnetic iron oxide nanoparticles are developing as promising candidates for biomedical applications such as targeted drug delivery. In particular, they represent an alternative to existing antitumor drug carriers, because of their ultra-fine size, low toxicity and magnetic characteristics. Nevertheless, there is a need to functionalize them in order to achieve good biocompatibility, efficient modification for further attachment of biomolecules, and improved stability. In this work we describe the functionalization of superparamagnetic maghemite nanoparticles encapsulated in a silica shell. After their chemical modification with positive (3-aminopropyl)trimethoxysilane, a gold layer was deposited in order to facilitate incorporation of the antitumor drug, doxorubicin (DOX), up to a maximum loading of 80μmol/g. In vitro cell uptake of nanocomposites was performed with DLD-1 colon cancer cells and PLC-PRF-5 liver cancer cells. Confocal microscopy photos illustrate that doxorubicin-loaded nanoparticles accumulate in both the cytoplasm and the cell nuclei. Cell survival efficiency with maghemite nanocomposites was determined via the MTT assay, and the cytotoxicity study proved that they exhibited significant toxicity against both types of cancer cells, although the improvement over free DOX treatment is more evident in the case of DLD-1 cancer cells when the most dilute drug and particle solutions are compared.