An efficient method for cleaning protein G affinity columns is described in which impurities are removed effectively with a combination of 4 mol/l urea and 0.1 mol/l sodium hydroxide. These cleaning conditions have only a minor deleterious effect on the binding capacity of the column and no effect on its selectivity for IgG. This cleaning procedure should prolong the life of costly affinity columns used in the purification of immunoglobulin G (IgG).