The isomer-free, amine-reactive Alexa Fluor 660 carboxylic acid, succinimidyl ester and its bright, far-red—fluorescent conjugates are excited by the 647 nm line of the krypton-ion laser or the 633 nm line of the He-Ne laser. A 5 mg unit size (A-20107) is also available. Fluorescence of this long-wavelength Alexa Fluor dye is not visible to the human eye but is readily detected by most imaging systems.