Virus inactivating chemicals and/or detergents in an aqueous composition containing a water-soluble plasma protein are reduced by selecting a suitable combination of temperature and concentration above 0.5M of salt with a high salting out effect according to the Hofmeister series, thereby forming vesicles containing the virus inactivating chemical and/or detergent. These vesicles are removed from the aqueous phase, e.g. by phase separation or filtration, and the protein thereafter isolated from the aqueous phase. The water-soluble plasma protein can be e.g. antithrombin III, transferrin or albumin. When the aqueous phase comprises e.g. a salt of citrate or sulphate in a concentration above 1M at room temperature, the reduction of virus inactivating chemical or detergent can be as high as 2000 times or more, giving a final concentration below 5 ppm.