Peripheral blood from 90 CLL patients was examined by light-and electron-microscopy for the occurrence of crystalline inclusions in lymphocytes. Inclusions were demonstrated in 10 patients (11%). In these patients the inclusions were present in 5-45% of peripheral blood lymphocytes. In the light microscope the inclusions appeared as rectangular, unstained structures in May-Grünewald Giemsa and PAS stains. In the electron microscope the inclusions appeared as intracytoplasmic, completely partially membrane-bound bodies, which were often associated with dilated profiles of rough endoplasmic reticulum. The ultrastructure of the inclusions was granular. In immunofluorescence staining the inclusions were found to contain immunoglobulin of the same type and class as the surface membrane-bound immunoglobulin of the neoplastic lymphocytes, most frequently IgM-lambda. The lymphocytes of one case with kappa light chains at the cell surface membrane contained inclusions of the same ultrastructural morphology as those of the other cases with lambda light chains. The presence of inclusions was not associated with any specific clinical or prognostic features. the inclusions persisted during antileukaemic therapy. Their formation may be related to a dysfunction in the synthesis of surface membrane-bound immunoglobulins.