The invention provides media and methods for culturing mammalian cells
whereby the sialylation of a protein produced by the cells is increased.
The medium can contain N-acetylmannosamine and, optionally, galactose.
The medium may also comprise fructose and mannose. Alternatively, the
medium can contain galactose and fructose and, optionally, can also
comprise mannose and/or N-acetylmannosamine. The methods can be practiced
along with other methods for culturing cells so as to increase the
quantity or quality of a protein produced by the cells, including
culturing the cells at a temperature below 37.degree. C.