Abstract: Process for the fed-batch culture of animal cells comprises culturing the cells in nutrient medium characterized in that during the culture the medium is supplemented with a combined feed of one or more energy sources and one or more animo acids, and culturing is continued into the decline phase of the culture to provide the product(s) of the cells. The process is particularly applicable to genetically modified cells, especially hybridoma cell cultures to produce monoclonal antibodies. Preferably, the supplemental feed is fed to the culture at a slow rate over a prolonged period. Very significant enhancement of overall product yield may be obtained.