Centrifugal ultrafiltration devices with 10K molecular weight cut-off (MWCO) membrane allow molecules with molecular weight smaller than 10,000 to pass through the membrane into the filtrate collection tube, while molecules larger than 10,000 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 antibody solutions, or to concentrate antibodies prior to conjugation. They also can be used to remove free dye or free biotin from antibodies 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 MWCO=10K are recommended for ultrafiltration of antibodies and other proteins larger than 30 kDa. Biotium also offers vials with MWCO=3K (catalog no. 22018) for ultrafiltration of proteins larger than 10 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.