An increasing number of novel molecular markers based on nanomaterials for tumor diagnostics have been developed in recent years. Many efforts have focused on the achievement of site-targeted bioconjugated nanoparticles. In contrast, the mechanisms of toxicity, endocytosis, and degradation pathways are still poorly understood, despite their primary importance for clinical translation. In this study, three different model nanoscale magnetofluorescent particle systems (MFNs) are designed and fabricated. These nanoparticles are evaluated in terms of size, morphology, zeta potential, fluorescence efficiency, capability of enhancing T(2) relaxivity of water protons, and stability. Accordingly, two are developed and the mechanism of internalization, the intracellular fate, and the toxicity in MCF-7 adenocarcinoma cells are studied. Besides the well-documented size effect, the anionic charge seems to be a crucial factor for particle internalization, as MFN penetration through the cell membrane could be modulated by surface charge. Ultrastructural analysis of transmission electron micrographs combined with evidence from confocal microscopy reveals that MFNs are internalized by clathrin-mediated endocytosis and macropinocytosis. Moreover, MFNs are found in EEA1-positive endosomes and in lysosomes, indicating that they follow a physiological pathway of endocytosis. Magnetorelaxometric analysis demonstrates that MFNs enable the detection of 5 x 10(5) cells mL(-1) after treatment with particle dosages as low as 30 microg mL(-1). Hence, MFNs appear to be a valuable and safe bimodal contrast agent that can be developed for the noninvasive diagnosis of breast cancer.