Mantle cell lymphoma (MCL) express immunoglobulin light chain lambda (IgL-lambda) more frequently than other non-Hodgkin's lymphomas, and IgL-lambda producing B-cells usually delete one or both alleles of their IgL-kappa genes. This inactivation is mediated by a rearrangement between the kappa deletion element (kappa de) and the Recombinant Signal Sequence (RSS) in the region between the Joining genes and the Constant region, or the RSS at the 3'-site of a Variable (Vkappa) segment. This deletion appears as a feasible tool for detecting monoclonality and minimal residual disease by polymerase chain reaction (PCR). Among twelve MCL patients studied, ten presented IgL-lambda expression, and all but one among these revealed a monoclonal kappa de rearrangement by PCR analysis. Six of the nine cases showed a fusion between the kappa de and the intron RSS, whilst three with a Vkappa segment. Since MCL has the worst prognosis of all B-cell lymphomas and high-dose chemotherapy regimens have been proposed, PCR for the kappa de rearrangement might be a useful molecular tool to evaluate the ability of the different treatment modalities to eradicate the malignant clones.