The isomeric mixture of the amine-reactive Alexa Fluor 633 carboxylic acid, succinimidyl ester and its conjugates yield fluorescence beyond the range of most sample autofluorescence. Excited by the 633 nm line of the He-Ne laser and the 635 nm line of red diode lasers, the Alexa Fluor 633 dye and its conjugates are bright and photostable, with excitation/emission maxima of ~632/647 nm. A 1 mg unit size (A-20005) 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.