The radial immunodiffusion method of Mancini has been applied to quantitative study of IgG subclasses in a normal population. The method was assessed in terms of both reproducibility and antiserum consumption. The distribution of IgG subclasses in a group of normal individuals was studied, and compared with their incidence in a series of monoclonal proteins investigated by previous workers and particularly with other quantitative studies on groups of normal individual's sera.