The invention relates to the removal of viruses from aqueous solutions, as a rule protein solutions, by ultrafiltration. This entails the viruses to be removed being increased in size by incubation with a high molecular weight receptor binding thereto, preferably a specific antibody, so that, on the one hand, the separation effect is improved and, on the other hand, a larger pore diameter which can now be chosen for the filters used also makes it possible for smaller viruses to be separated from larger protein molecules present in protein solutions, and, where appropriate, the filtration rate is increased.