The aggregated size distribution of the γ-globulin and albumin in aqueous solution was analyzed using flow cytometry, which showed reproducible data compared to conventional light scattering measurements. The addition of NaCl to protein solution effectively reduced the aggregated sizes of proteins in the solution. DNase treatment in the protein solution was also found to effectively reduce the size of aggregated proteins. Microfiltration of an albumin solution was investigated through regenerated cellulose virus removal membranes with different nominal pore sizes (r=15, 35 and 75 nm). Membranes with 35 nm nominal pore sizes were effective in enhancing the flux of the microfiltration of an albumin solution containing NaCl or containing NaCl and treated with Micrococcal nuclease (DNase). This is thought to be due to the similar size of the pores of the Planova 35N membranes and the small aggregated albumin. DNA rejection was investigated by evaluating the permeation of albumin solutions containing 0 or 0.15 mol/l NaCl and/or treated with 15 units of Micrococcal nuclease. The addition of NaCl and/or treatment of Micrococcal nuclease was effective in reducing the DNA concentration in the albumin solution after microfiltration of the solution using r=15 nm membranes as compared to using membranes with larger pore sizes (r=35 and 75 nm).