Treatment of light chain (LC) deposition diseases both nonfibrillar and fibrillar is aimed at eliminating LC production but success is limited. We report on the testing of an small interfering RNA pool targeting the κ LC constant region mRNA (si[IGKC]) designed for use against all κ plasma cell clones. To test for changes in κ LC message and protein production we used real-time PCR, immunoblot, intracellular mean fluorescence intensity and κ LC secretion by enzyme-linked immunosorbent assay. In vitro we employed 4 human cell lines that make κ LCs and 20 specimens of CD138-selected marrow plasma cells from patients with κ plasma cell diseases. In vivo, we used a murine flank plasmacytoma xenograft model. In vitro and in vivo, there were significant reductions in message and protein production by all modalities in all cell types despite diversity in variable region sequence. In addition, in clones producing intact immunoglobulin, caspase 3/7 activity with si[IGKC] was significantly increased compared with clones producing κ LC only, consistent with the triggering of a terminal unfolded protein response by excess unpaired heavy chains. In conclusion, si[IGKC] can significantly reduce κ LC production by κ plasma cells. Further preclinical development is needed to optimize delivery.