Conjugates prepared with the thiol-reactive Alexa® Fluor 660 C₅ maleimide are optimally excited with the 647 nm spectral line of the krypton-ion laser and well excited by the 633 nm spectra line of the He-Ne laser and produce bright far-red fluorescence emission, with a peak at 690 nm. The wide separation of its emission from that of other fluorophores allows use of Alexa Fluor® 660 dye with other fluorescent labels, including Alexa Fluor® 555 and Cy3 dyes and phycoerythrin (R-PE) conjugates.