The aim of this work was to develop and test the in vitro biological activity of nanocapsules loaded with a doxorubicin (DOX) free base dissolved in a core of castor oil shelled by poly(methyl vinyl ether-co-maleic anhydride) conjugated to n-octadecylamine residues. This system was stable and monodisperse, with a hydrodynamic diameter of about 300 nm. These nanocapsules changed the intracellular distribution of DOX, from the nuclei to the cytoplasm, and exhibited higher toxicity towards cancer cells - 4T1 and MCF-7 - and significantly lower toxicity towards normal cells - NIH-3T3 and MCF-10A - in vitro. In conclusion, these nanocapsules are suitable DOX carriers, which remain to be studied in in vivo tumor models.