Microfiltration of a gamma-globulin solution has been investigated through the virus removal membranes having different pore sizes (i.e. r = 15, 35 and 75 nm) and a dialysis membrane (r = 3.4 mm), which were all made of the same regenerated cellulose material. The addition of NaCl in the gamma-globulin feed solution was effective to enhance the flux and transmission through the membranes having a pore size ranging from 15 to 75 nm. DNase treatment of a gamma-globulin solution with Micrococcal nuclease enhanced the flux and transmission of gamma-globulin through the membranes either with or without NaCl. The membranes having a pore size of 35 nm showed dramatically enhanced flux in the microfiltration of a gamma-globulin solution containing NaCl and/or being treated with Micrococcal nuclease. This can be explained as a DNase treatment and NaCl addition in the protein solution dissociate protein aggregates of DNA-gamma-globulin complex, which plugs the pores in the microfiltration membranes.