Method of preparing a protein solution c virally safe by a dual stage nanofiltration

The present invention relates to a method of preparing a solution of protein C virally secure, comprising: a) providing a solution of protein C purified beforehand, b) a first stage of nanofiltration of said protein solution C previously purified through a nanofilter having an average pore size comprised between 15 and 35 nm, and c) a second nanofiltration step of the protein solution C obtained at the end of the first nanofiltration through a nanofilter having a second average pore size less than or equal to 20 nm, advantageously less than or equal to 15 nm. It also relates to a solution of protein C virally secure obtainable by this process and its use as medicament, particularly in the substitution therapy in congenital or acquired deficiency of protein C.