This study compared the quantitation of immunoglobulins and light chains with qualitative analysis by immunoelectrophoresis and immunofixation to determine the usefulness of these techniques for the detection of monoclonal gammopathy. Emphasis was on the recently available nephelometric technique for light chain quantitation. Quantitation of immunoglobulins and light chains correctly typed 142 of 159 cases of monoclonal gammopathy. False-negative results were only seen in 17 cases of minimonoclonal gammopathies (IgG-kappa, IgG-lambda, and kappa light chain), in which either the quantitation of light chains or of immunoglobulins, or the kappdlambda ratio remained normal. Among 92 control cases, abnormal kappa/ lambda ratios were seen in three AIDS patients, two cases of polyclonal gammopathy, and one case of hypogammaglobulinemia, but none of them were misdiagnosed as monoclonal gammopathy by quantitation. For clinical laboratories, quantitative assays have some advantages over qualitative analysis because they are less demanding with respect to technique and interpretation, and they allow earlier result reporting and discernment in patients' follow-ups. Qualitative analysis should be reserved for equivocal cases.