The physicochemical characterization of nanoparticles in suspension is a prerequisite for the adequate assessment of their potential biological effect. Little is known to date about the colloidal stability of TiO2 nanoparticles in cell culture medium. This study investigates the effect of particle concentration, ionic strength, pH, and the presence of fetal bovine serum (FBS) and human serum albumin (HSA) on the colloidal stability of TiO2 nanoparticles in RPMI cell culture medium, by sedimentation measurements, dynamic light scattering, and electrokinetic measurements (zeta-potential). TEM revealed that the particles were polydisperse, with diameters ranging from approximately 15 to approximately 350 nm. The agglomeration rate and sedimentation rate increased with particles' concentration. The size of the agglomerates at 100 mg/L TiO2 was significantly reduced, from 1620+/-160 to 348+/-13 and 378+/-15 nm, upon the addition of 10% (v/v) FBS and 1% (w/w) HSA, respectively. The isoelectric point of TiO2 in water was 2.9 and the measured zeta-potential in RPMI was -16+/-2 mV at pH 7.4. A slight increase in the zeta-potential of TiO2 in RPMI was observed upon the addition of FBS and HSA. The addition of FBS and HSA prevented high agglomeration, leading to a stable dispersion of TiO2 nanoparticles for at least 24 h, possibly due to steric stabilization of the particles.