Most common plasma cell tumor is -

Multiple myeloma is a malignant proliferation of plasma cells. In multiple myeloma,plasma cells produce immunoglobulin of a single heavy & light chain,a monoclonal protein commonly referred to as paraprotein.In some cases,only light chain is produced & this appears in the urine as Bence Jones protein .Reference:Davidson's principles & practice of medicine 22nd edition pg no 1046.