Flow-Check Fluorospheres consist of 10 μm (nominal diameter) polystyrene fluorescent microspheres suspended in an aqueous medium containing surfactants and preservatives at 1 x 106 fluorospheres/mL (nominal concentration). The fluorescence emission of the dye contained within the fluorospheres ranges from 525 nm to 700 nm when excited at 488 nm. Flow-Check Fluorospheres are used to verify instrument optical alignment and fluidics.