A simple method of stabilization of detonation nanodiamonds in isotonic aqueous-saline media was found, being a solution of an actual task for biomedical applications. The stable colloid of detonation nanodiamond particles with negative ζ-potential in isotonic medium can be produced by complexes formation with poly(vinylpyrrolidone). The mean size of the complexes is 30–35 nm. The stability conditions of the complexes were defined and their structure was determined by small-angle neutron scattering. The obtained hydrosols of nanodiamond particles are stable in physiological medium and can be used in biological researches and in medicine as drug carriers.