The properties of an Fc-containing protein, for example, an antibody, are controlled by altering the sialylation of the oligosaccharides in the Fc region by transfecting the cell line expressing the Fc-containing protein with a vector sequence encoding a sialidase. The modified Fc-containing proteins have therapeutic utility in diseases or conditions in which it is desirable to control the affinity for one or more of the FcγRI, FcγRIIA, and FcγRIIIA receptors, ADCC activity, macrophage or monocyte activation, serum half-life, and avidity.