Permeation of gamma -globulin solution through three microfiltration membranes (Planova 35N (mean pore size (r) = 0.035 mum). Nucleopore(r = 0.1 mum) and Fluoropore(r = 0.1 mum)) was investigated. A drastic increase in flux was found with a decrease in DNA in the feed solution, which was normally contained in the protein solution as a contamination in the order of ppb. A DNA-gamma -globulin complex having a particle size of 135-225 nm was detected by dynamic light scattering measurements in the gamma -globulin solution, The DNA-gamma -globulin complex was destroyed by the addition of 0.4 mol/l NaCl in the gamma -globulin solution. The flux of the gamma -globulin solution through Planova 35N membranes drastically increased with the increase in NaCl concentration in the gamma -globulin solution up to 0.1 mol/l. This is explained bq: a decrease in the DNA-gamma -globulin complex in the presence of NaCl in the gamma -globulin solution. Only a slight increase in the flux of the gamma -globulin solution through Nucleopore membranes was found, on the other hand, a decrease in flux through the Fluoropore membranes was observed when the NaCl concentration in the feed solution of gamma -globulin increased. These results suggest that the effect of flux on the concentration of NaCl depends on the pore sizes and structure of the membranes and membrane materials,