Fluorescence and electron microscopy studies were carried out on the blood and bone marrow cells of the first patient observed to have µ-chain disease. Since previously reported cases of γ and α heavy-chain disease did not elaborate any light chains, it was of interest to determine whether both the µ-chain fragments and the κ chains found in the serum of this patient originated in the same cell or whether mutations had affected two different clones. The use of rhodamine-conjugated and fluorescein-conjugated antisera to heavy and light chains respectively established that both antigenic determinants were present in the same cell. On electron microscopy, the plasma cells showed large vacuoles which appeared to form in the vicinity of the Golgi zone and frequently extended to the surface of the cell. It is postulated that the structural defect in the µ chain interferes with proper assembly of the gamma-globulin molecule. This in turn may preclude transport via the normal secretory pathway. The accumulated protein fragments may be released by a process of limited cytolysis.