The increased sensitivity of immunofixation electrophoresis (IFE) over prior electrophoretic methods has led to renewed interest in the study of free light chains. Here, we discuss problems associated with the identification of monoclonal free light chains (Bence Jones proteins) in urine. Besides reviewing the nature of the sample specimens and the assays themselves, we discuss the physiology, biochemistry, genetics, and immunological properties of these molecules. Direct measurement of kappa/lambda ratios may ultimately be useful, but all commercial methods available now lack sufficient sensitivity. IFE is the preferred method because of its sensitivity and ease of interpretation. There are, however, difficulties associated with the interpretation of urinary IFE patterns, because the technique does not include an intrinsic mechanism for antibody-antigen titration and because of its great sensitivity in the absence of quantification. Problems of interpretation are discussed.