To lower the titer of a virus and reduce the danger of viral infection in the treatment of human disease without giving undesirable effect on the activity of proteins by filtering a solution containing biologically active proteins under specific condition.

A solution containing biologically active proteins is filtered with a depth filter having an average pore diameter larger than the average diameter of the virus to be removed, a cationic charge and a distinct structure at a pH of about 4.5-8.0, preferably about 5.0-7.0. It is necessary to filter the solution under a condition to obtain a filtered solution having essentially decreased virus titer while keeping the biological activity of the proteins.