To limit cytotoxicity of anticancer drugs against healthy cells, an appropriate carrier should be synthesized to deliver the drug to the tumor tissue only. A good solution is to anchor a magnetic nanoparticle to the molecule of the drug and to use a properly directed external magnetic field. The synthesis of the conjugate of doxorubicin with magnetic nanoparticles (iron oxide) modified by us resulted in a substantial depression of the aggregation process of the nanoparticles and therefore allowed the correct examination of cytotoxicity of the modified drug. It has been shown, by performing the electrochemical microbalance measurements, that the use of magnetic field guaranteed the efficient delivery of the drug to the desired place. The change in the synthesis procedure led to an increase in the number of DOX molecules attached to one magnetic nanoparticle. The release of the drug took place at pH 5.8 (and below it), which pH characterizes the cancer cells. It has also been found that while the iron oxide magnetic nanoparticles were not cytotoxic toward human urinary bladder carcinoma cells UM-UC-3, the tumor cell sensitivity of the DOX-Np complex was slightly higher in comparison to the identical concentration of doxorubicin alone.