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.