Abstract: A multi-step process for purifying an immune serum globulin fraction from a crude plasma protein fraction involves precipitating non-serum globulin proteins from an aqueous suspension of the crude plasma protein fraction using a protein precipitant, adding a virus-inactivating agent to the clarified immune serum globulin-containing liquid, absorbing the immune serum globulins onto a cation exchange resin and washing non-serum globulin contaminants from the resin, subjecting the eluate to ultrafiltration to concentrate the immune serum globulins and separate them from low molecular weight species, contacting the concentrate with an anion exchange resin to absorb non-serum globulin contaminants, passing the imune-serum globulins through the anion exchange resin under conditions that leave non-serum globulin contaminants bound to the resin, and subjecting the filtrate to a molecular washing step to produce a purified immune serum globulin fraction.