Ultrafiltration can be used to rapidly and easily remove small molecules such as Tris, glycerol, glycine etc. from antibody solutions, to concentrate antibodies before or after conjugation, or remove free dye or biotin from antibodies after conjugation. The ultrafiltration membrane retains molecules with molecular weight (MW) larger than the cut-off, allowing smaller molecules to pass through into the filtrate collection tube. We offer centrifugal ultrafiltration devices with two MW cut-off options: MWCO=10 kD (10K) and MWCO=3 kD (3K). 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 proteins larger than 30 kDa. Vials with MWCO=3K are recommended for ultrafiltration of proteins larger than 10 kDa.Ultrafiltration is not effective for fractionating proteins (based on size) unless there is at least a 10-fold difference in MW.