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Free kappa L chains (FkappaLCs) are expressed on the surface of myeloma cells and are being assessed as a therapeutic target for the treatment of multiple myeloma. Despite its clinical potential, the mechanism by which FkappaLCs interact with membranes remains unresolved. In this study, we show that FkappaLCs associate with sphingomyelin on the plasma membrane of myeloma cells. Moreover, membrane-bound FkappaLCs are aggregated, suggesting that aggregation is required for intercalation with membranes. Finally, we propose a model where the binding of FkappaLCs with sphingomyelin on secretory vesicle membranes is stabilized by self-aggregation, with aggregated FkappaLCs exposed on the plasma membrane after exocytosis. Although it is well known that protein aggregates bind membranes, this is only the second example of an aggregate being found on the surface of cells that also secrete the protein in its native form. We postulate that many other aggregation-prone proteins may associate with cell membranes by similar mechanisms.