The isomer-free, amine-reactive Alexa Fluor 555 carboxylic acid, succinimidyl ester and its conjugates have spectra identical to that of Cy3 dye. The conjugates are readily excited by the 546 nm line of the mercury-arc lamp and the 543 nm line of the He-Ne laser. A 1 mg unit size (A-20009) is also available.