Before the widespread availability and use of Protein A (rabbit) or Protein G (rodent) for the purification of IgG, the use of an ammonium sulfate “cut” was the standard method to isolate IgG and other serum proteins. The addition of ammonium sulfate reduces the effective solubility of proteins through direct competition for binding sites on the surface of the protein. The resulting precipitated proteins can be isolated by centrifugation. An ammonium sulfate concentration between 40% and 50% results in the precipitation of IgG from most species, and thus 50% is usually used. Because other proteins can be “trapped” within the aggregated protein, the use of ammonium sulfate does not result in a purified antibody fraction and as such should be considered a first step in a multistep antibody purification protocol.