To manufacture a glycoprotein, mammalian cells expressing the desired protein are often grown in fed-batch mode. Feeding an undefined, nonanimal hydrolysate helps the cells receive sufficient nutrition, but makes systems difficult to optimize. Even different lots of the same hydrolysate may have significant variability; furthermore, individual components may actually be detrimental to the cells. Switching to fully defined feeds could eliminate these issues. For monoclonal antibody (mAb) production by fed-batch NS0 cells, this article describes the replacement of a hydrolysate-based feed with a fully defined, animal-component-free feed system. The defined feed initially had 67 components, but additional experiments allowed a reduction to 25 components. The mAb titer is approximately 20% higher than in the undefined system, and the feed volume is circa 20% lower. The two systems generated antibodies with similar glycosylation profiles. Other benefits of the defined feed system include lower raw material costs, the ability to optimize key nutrient concentrations, greater confidence in raw material quality, and the elimination of potential, hydrolysate-associated endotoxin issues. (C) 2010 American Institute of Chemical Engineers Biotechnol. Prog., 26: 1411-1416, 2010