CF350 streptavidin conjugate is prepared from high quality streptavidin and the fluorescent dye CF350. CF350 (Ex/Em = 347/448 nm) is a blue fluorescent dye spectrally similar to the traditional blue fluorescent dye, AMCA. However, compared to AMCA, CF350 is more water-soluble, more photostable and at least 50% more fluorescent on proteins. CF350 is at least as bright as Alexa Fluor? 350 and thus can be a direct replacement for the latter for all applications.