This study is the third part of a multicenter evaluation carried out with ten analysers. Five proteins (IgG, IgA, IgM, transferrin, haptoglobin) were assayed in three sera, each containing one monoclonal Ig (IgG, IgA, or IgM). The expected agreement was not obtained with these particular sera, except in the case of haptoglobin. The marked imprecision of the monoclonal Ig quantification is highlighted, including the fact that many erroneous results are due to the antigen excess phenomenon. It is also shown that in the serum containing monoclonal IgM, quantification of the other proteins, remaining polyclonal Ig, as well as transferrin and haptoglobin, is difficult. The authors recall that electrophoresis and immunoelectrophoresis, which are indispensable for the characterization of the monoclonal components, are of great help in solving such difficulties.