The thiol-reactive Alexa Fluor 594 C₅ maleimide and its conjugates exhibit very little spectral overlap with green-fluorescent conjugates and are efficiently excited by the 568 nm line of the Ar-Kr laser and by the 594 nm line of the orange He-Ne laser. Spectrally similar to Texas Red dye, the Alexa Fluor 594 dye and its conjugates exhibit bright fluorescence and have high photostability.