The isomeric mixture of amine-reactive Texas Red-X, succinimidyl ester and its conjugates emit at a longer wavelength than do either tetramethylrhodamine or Lissamine rhodamine B, and are among the most commonly used long-wavelength “third labels” in fluorescence microscopy and flow cytometry. Protein conjugates prepared with Texas Red-X succinimidyl ester also have a higher fluorescence yield than those with the same labeling ratio prepared with Texas Red sulfonyl chloride.