Rearrangement of the light chain locus is believed to be an ordered process in which Iglambda rearrangements only occur if Igkappa rearrangements are found to be non-productive or self-reactive. Secondary rearrangements of the B-cell receptor (BCR) have shown, however, that rescue of abortive Igkappa rearrangements or autoreactive B cells can be achieved through receptor editing using upstream V-regions as the template sequences. Since secondary rearrangement can occur in the periphery, possibly in a subset of B cells maintaining constitutive Rag activity, it is conceivable that two light chains (kappa:kappa or kappa:lambda) could be expressed in these cells, apparently in violation of allelic exclusion. Previously, we have reported that silicone-induced plasmacytomas (SIPCs) exhibit dual expression and ongoing rearrangements of Igkappa and Iglambda. In this paper, we show by ELISA that both Igkappa and Iglambda are found at the protein level, but are secreted in different amounts. Furthermore, we demonstrate by micro-manipulation and RT-PCR amplification that Igkappa and Iglambda are simultaneously expressed in a single SIPC cell. We propose that these dual-expressing cells, found intermittently in cases of plasmacytomas (PCs), may have originally been immature B cells when transformed but now are maintained as a long-lived mature B cell found infrequently in the tumor population.