We hypothesized that using a free light chain (FLC) assay as an adjunct to capillary zone electrophoresis (CZE) could improve detection of lymphoplasmacytic processes. We prospectively studied 1,003 consecutive serum samples submitted for routine protein electrophoresis and/or immunofixation electrophoresis by CZE and FLC. Samples from patients previously characterized as having M proteins were excluded. Protein electrophoresis was read by a pathologist unaware of the FLC results. Sixteen cases revealed an abnormal free kappa/lambda ratio in which CZE did not demonstrate an M protein. Nine cases of B-lymphocyte or plasma cell proliferative processes were detected by an abnormal free kappa/lambda ratio in which CZE did not demonstrate an M protein. Cases with low free kappa/lambda ratios included 1 chronic lymphocytic leukemia (CLL), 1 IgM lambda with aplastic anemia, and 1 lambda light chain myeloma. Cases with high free kappa/lambda ratios included 2 CLL, 1 lymphocytosis (possibly early CLL), 1 kappa light chain myeloma, 1 atypical lymphoma with neuropathy, and 1 nonsecretory myeloma. Addition of the free kappa/lambda ratio to CZE increases the yield of lymphocyte and plasma cell proliferative processes detected by 56%.