The dissolution of silver nanoparticles (10 nm) is strongly enhanced by the presence of platinum nanoparticles (3 nm) in chloride-containing aqueous dispersion. This was shown using X-ray powder diffraction and transmission electron microscopy. Direct contact between the two metals is necessary as shown by transmission electron microscopy. Complete dissolution of silver nanoparticles occurs within about one hour in the presence of potassium chloride as the electrolyte but not in pure water. Thus, the dissolution of silver nanoparticles in aqueous dispersion requires the presence of platinum nanoparticles for polarization as well as the presence of electrolyte ions for charge balancing. After dissolution, the silver ions precipitate as silver chloride. A minor part of silver is taken up by platinum to form a solid solution (alloy). The enhanced release of silver ions in the dispersion of nanoparticles of both metals leads to a decreased viability of human mesenchymal stem cells (hMSC), caused by the cytotoxic effect of silver ions.