Simultaneous micro and ultrafiltration were used to recover a recombinant monoclonal antibody from mammalian cell culture. The yield of the microfiltration was affected by the choice of the base used to control the pH in cell culture. A higher cell density and titre could be maintained with sodium hydroxide, while stable viability and a higher sialic ratio were obtained with sodium carbonate. Lysis, rejection coefficient and resistance (ratio of trans-membrane pressure to penetration velocity) were internally consistent. The MF resistance was higher for lots of sodium hydroxide. Product retained by de-sialylated MF membranes with increased lysis during operation, while the product in the MF permeate retained its glycosylation level. MF membranes also retained multimeric forms of the product, which are known to have lower sialylation. The MF permeate was concentrated on UF membranes and then filtered through a polishing filtration stream prior to downstream processing. DNA and protein aggregates were found to increase in size during MF / UF operation and during UF concentration. The presence of such aggregates adversely affected the polishing filtration. It was found that borosilicate glass and polyetherether sulfone membranes pre-filter this material better than nylon, mixed esters of cellulose and polypropylene membranes. The yield of the nylon sterilization grade membranes (0.2 μm) was better than the PVDF membranes. In addition, the performance of PVDF membranes was different for different manufacturers. Increasing the pH of the UF concentrate improved polishing filtration for some batches. Presence of bio-load and air entrainment / foam formation complicated filtration polishing.