Immunoelectrophoresis and immunofixation are not successful in determining the light chain type of some monoclonal immunoglobulins having configurational changes in their light chain structure (“unreactive” light chains) which prevent precipitation reaction with anti-light chain sera. An immunoselection technique was used to ascertain the light chain type of two such monoclonal immunoglobulins of IgD and IgA classes. It appears that a co-precipitation reaction in this assay circumvents the problem of “hidden” determinants. This technique is simple, reproducible, and could be used to characterize monoclonal immunoglobulins that do not precipitate with anti-light chain sera.