Silica microspheres coated with a silica or titania shell containing fluorescent semiconductor nanocrystals are described. Since the coating process does not affect the monodispersity of the silica microspheres, microspheres of a defined size and emission wavelength, potentially useful for biological applications, can be fabricated. The Figure shows 100 nm diameter blue-light-emitting and 500 nm diameter red-light-emitting microspheres circulating in the brain of a mouse.