Synthesis and characterization of Co-SPION. Co-SPIONs were synthesized in the thermostated glass reactor by the Massart co-precipitation method (Massart, 1981) from the alkaline solutions of Co(II) and Fe(III) metal salts at 70 °C for 5 h. All reagents in the synthesis procedure were at least analytical grade and were used without any further purification, except NaOH, which was purified by preparation of a saturated solution leading to the crystallization of other sodium salts. CoCl2, Fe2(SO4)3, and citric acid were purchased from Aldrich Chemicals Inc. Ultrapure water was used during all experiments. For the preparation of working solutions 0.12 mol/L CoCl2, 0.06 mol/L Fe2(SO4)3, 5.0 mol/L NaOH and 0.3 mol/L citric acid were prepared and deoxygenated with argon before mixing. The molar ratio of cobalt(II) and iron(III) salts in the reactor was 1:1.2 at their total concentration - 40 mmol. The pH of the solutions was maintained at 11.5. The required amount of 5.0 mol/L NaOH was determined by an additional blank experiment. In the next experiment, the estimated amount was added to the reactor, containing all other components, over several seconds under vigorous stirring. The synthesis in the thermostated reactor was carried out under continuous bubbling of argon gas. The raw products were centrifuged at 7500 rpm for 5 minutes and rinsed several times. The supernatants obtained from the last three centrifugations were combined, neutralized by addition of citric acid solution to pH 6.0, and used as a stable ferrofluid within the following week. The composition of the synthesized products was studied by energy-dispersive -aminoactinomycin D (BD Biosciences, USA). Magnetically activated cell sorting. Magnetically activated cell sorting (MACS) was used to separate magnetically labeled and unlabeled cells after 48 h of incubation with Co-SPION. A single-cell suspension of treated cells was passed through an LS column, placed in a MidiMACS separator (Miltenyi Biotec, Germany). The fraction not magnetically labeled flew freely through the column. The magnetically labeled fraction was collected by removing the LS column from the magnetic field and eluting the attached cells with PBS. Both fractions were examined under the microscope and cell counter to verify the presence of cells. Statistical analysis. Triplicate measurements of each sample were performed in three independent experiments. Statistical differences between groups were analyzed using Student's t test. A value of p