1. Prepare saturated ammonium sulfate ( (NH₄)₂SO₄, ~ 1kg/liter ), autoclave.

2. Add equal volume of saturated ammonium sulfate to antibody sample. Stir at 4 °C overnight.

3. Spin down precipitates (e.g., 3,500 rpm for 30 minutes, or shorter time with higher speed).

4. Discard supernatant. And resuspend the pellet in PBS.

5. If this is the final step in antibody purification, dialysis against PBS.