Nanoparticles are being increasingly used in consumer products worldwide, and their toxicological effects are currently being intensely debated. In vitro tests play a significant role in nanoparticle risk assessment, but reliable particle characterization in the cell culture medium with added fetal bovine serum (CCM) used in these tests is not available. As a step toward filling this gap, we report on silver ion release by silver nanoparticles and on changes in the particle radii and in their protein corona when incubated in CCM. Particles of a certified reference material, p(1), and particles of a commercial silver nanoparticle material, p(2), were investigated. The colloidal stability of pi is provided by the surfactants polyethylene glycol-25 glyceryl trioleate and polyethylene glycol-20 sorbitan monolaurate, whereas p(2) is stabilized by polyvinylpyrrolidone. Dialyses of p(1) and p(2) reveal that their silver ion release rates in CCM are much larger than in water. Particle characterization was performed with asymmetrical flow field-flow fractionation, small-angle X-ray scattering, dynamic light scattering, and electron microscopy. p(1) and p(2), have similar hydrodynamic radii of 15 and 16 nm, respectively. The silver core radii are 9.2 and 10.2 nm. Gel electrophoresis and subsequent peptide identification reveal that albumin is the main corona component of p(1) and p(2) after incubation in CCM that consists of Dulbecco's modified Eagle medium with 10% fetal bovine serum added.