Distinctive morphological features in both the marrow infiltrate and the kidney were seen in a 52-year-old woman with kappa light chain-producing plasma cell myeloma, diagnosed on the basis of multiple osteolytic lesions, the presence of atypical plasma cells in the bone marrow, and monoclonal immunoglobulin production as demonstrated by immunoperoxidase staining on marrow sections. Large focal collections of histiocytes in the bone marrow and the renal proximal tubular epithelium had abundant glassy cytoplasm. Characteristic crystalline inclusions were seen ultrastructurally in both types of cells. It is believed that these crystalline deposits are lysosomal inclusions composed of altered kappa light chains taken up by these cells. The renal changes were entirely different from those of myeloma kidneys and were associated with proximal tubular dysfunction of adult Fanconi syndrome without distal tubule abnormality.