A method for separating off viruses from a protein solution by nanofiltration, comprising adding to the protein solution at least one ingredient chosen from chaotropic substances chosen from arginine, guanidine, citrulline, urea and derivatives thereof and salts thereof, and compounds chosen from polyethoxysorbitan esters and derivatives thereof, prior to the nanofiltration, in order to decrease or prevent aggregation of the protein molecules, and then filtering the solution through a filter having a pore size ranging from 15 nm to less than 35 nm.