These Living Colors recombinant proteins are purified from E. coli. Each recombinant variant has absorption and emission spectra identical to those for the expressed proteins. The purified protein retains its fluorescence capability under many harsh conditions, including high heat, pH extremes, exposure to chaotropes, and incubation with low concentrations of detergents. These protein are suitable as standard in Western blots and protein gels. They can also be used as calibration standards for fluorometers and flow cytometry instruments.