The selective delivery of the steroid anti-estrogen RU 58668 (RU) to human breast cancer MCF-7 cells by using nanoscaled hybrid liposomes, labelled with rhodamine, coated by poly(ethylene glycol) and entrapping superparamagnetic nanocrystals of maghemite (gamma-Fe2O3) was investigated. Stable drug incorporation within the vesicle bilayer was ascertained by differential scanning calorimetry and small-angle X-ray diffraction. The study of the interactions with living tumor cells of the magnetic-fluid-loaded liposomes containing RU molecules (RU-MFLs) showed a significant improvement in RU-MLF uptake through their guidance by a magnetic field gradient of 155 T m(-1) produced by a 0.44 T external magnet placed in the near vicinity of the cells. RU-MFLs enter the cells by endocytosis, as revealed by electron microscopy, and greatly accumulate therein as intact structures, as supported by confocal fluorescence microscopy images, which revealed a lipid vesicle pathway, as well as by independent iron oxide quantification by magnetophoresis or electron spin resonance. Inhibition assay of estradiol-induced transcription of the luciferase reporter gene in MELN cells confirmed that RU anti-estrogenic activity was entirely preserved upon its liposome-mediated intracellular uptake. This work provides a proof of concept of intracellular magnetic targeting of a therapeutic substance by means of superparamagnetic liposomes, which is especially promising as a tool for cancer therapy.