Crude immunoglobulin G isolated from human blood plasma is treated according to a conventional technique (such as the tricalcium phosphate adsorption method) to remove aggregates therefrom to such an extent that they are not detectable by gel filtration analysis. In order to produce an aqueous solution of immunoglobulin G having a reduced anticomplementary activity, the resulting solution is then filtered through a porous polyolefin membrane having a pore size larger than the molecular size of immunoglobulin G, in the presence of a stabilizer having surface activity. The aqueous solution of immunoglobulin G so produced is suitable for use in intravenous injection because its anticomplementary activity is low.