Centrifugal ultrafiltration devices with 3K molecular weight cut-off (MWCO) membrane allow molecules with molecular weight smaller than 3,000 to pass through the membrane into the filtrate collection tube, while molecules larger than 3,000 molecular weight are retained on top of the ultrafiltration membrane. Ultrafiltration can be used to rapidly and easily remove small molecules such as Tris, glycerol and glycine from protein solutions, or to concentrate proteins prior to conjugation. They also can be used to remove free dye or free biotin from proteins after conjugation. For ultrafiltration of proteins, it is recommended to use a MWCO 3- to 6-times smaller than the protein to be retained. Ultrafiltration vials with with MWCO=3K are recommended for ultrafiltration of proteins larger than 10 kDa. Biotium also offers vials with MWCO=10K (catalog no. 22004) for ultrafiltration of antibodies and other proteins larger than 30 kDa. Ultrafiltration is not effective for fractionating proteins (separating one protein from another protein based on size) unless there is at least a 10-fold difference in the protein molecular weights.