The isomeric mixture of amine-reactive Alexa Fluor 568 carboxylic acid, succinimidyl ester and its conjugates are excited by the 568 nm line of the Ar-Kr laser and yield bright, orange-red fluorescence, brighter than that seen with Lissamine rhodamine B or Rhodamine Red-X conjugates. The exceptionally photostable conjugates of the Alexa Fluor 568 dye exhibit less spectral overlap with green-fluorescent conjugates than do orange-fluorescent conjugates. Also available in a 1 mg unit size (Catalog number: A-20003).