In line ultrafiltration (UF) can significantly increase the recoverable mass of biopharmaceutical products when pool tank volumes are limiting. Using relatively small commercially available ultrafiltration cassettes, a proof-of-concept study demonstrates that inline UF can significantly increase recoverable mass in an antibody purification process. With ever-increasing cell culture titers pushing product masses to higher levels, inline UF offers a relatively easy-to-implement and less disruptive alternative to installing larger pool tanks and enables more cost-effective production utilizing facilities designed for smaller bulk sizes. (C) 2010 American Institute of Chemical Engineers Biotechnol. Prog., 26: 1068-1072, 2010