We report the evaluation of cytotoxicity of a new type of engineered nanomaterials, FePt@CoS2 yolk-shell nanocrystals, synthesized by the mechanism of the Kirkendall effect when FePt nanoparticles serve as the seeds. The cytotoxicity of FePt@CoS2 yolk-shell nanocrystals, evaluated by MTT assay, shows a much lower IC50 (35.5 +/- 4.7 ng of Pt/mL for HeLa cell) than that of cisplatin (230 ng of Pt/mL). In the control experiment, cysteine-modified FePt nanoparticles exhibit IC50 at 12.0 +/- 0.9 mu g of Pt/mL. Transmission electron microscopy confirms the cellular uptake of FePt@CoS2 nanocrystals, and the magnetic properties analysis (SQUID) proves the release of FePt nanoparticles from the yolk-shell nanostructures after cellular uptake. These results are significant because almost none of the platinum-based complexes produced for clinical trials in the past 3 decades have shown higher activity than that of the parent drug, cisplatin. The exceptionally high toxicity of FePt@CoS2 yolk-shell nanocrystals (about 7 times higher than that of cisplatin in terms of Pt) may lead to a new design of an anticancer nanomedicine.