Avidin DCS, Fluorescein labeled, is a cell sorter grade of Fluorescein Avidin D. Through the development of a new method of fluorescein labeling we are able to incorporate more fluorochrome into a protein without altering the properties of the protein. Not only does the Fluorescein Avidin DCS have an optimal F/P ratio, the efficiency of fluorescence is also increased. In addition to the improved conjugation technique, we employ a purification step to reduce nonspecific binding which can hamper resolution in cell sorting and other applications. Although this product has been designed for cell sorting, it can also be used in fluorescent microscopy when higher sensitivity and lower background staining are necessary. This derivative is preferred when using the amplification procedure with Biotinylated Anti-Avidin D. Fluorescein Avidin DCS has an excitation at 495 nm and an emission at about 515 nm.