Methods for producing biological solutions such as immunoglobulins and in particular anti-D immunoglobulin substantially free of abnormal prion protein resulting therefrom. Specifically provided are methods for aggregation of prions and depth filtration of the biological solution to capture and remove abnormal and if desired, normal prion protein. The prion protein may then be eluted from the depth filter and filter washes and concentrated sufficient for detection at limits currently required by available assays.