Abstract: An antigen/antibody precipitate is obtained, using monoclonal antibodies, the monoclonal antibodies (samples I or II or III or IV) being selected so as to be specific to two distinct antigenic binding sites (L or C 2 or C 3) on a protein (IgG) in a sample under test. The proportions of sub-populations of immunoglobulins (IgG kappa, IgG lambda) in a sample is determined by reacting the sample with a combination of antibodies (II and IV) both of which are specific to the heavy chains (H) of both sub-populations (IgG kappa, IgG lambda) and reacting the sample with an antibody combination (I and II) specific to said heavy chain (H) and to an antigenic determinant expressed by only one (IgG kappa) of the sub-populations.