Five patients with a serum M component were shown to possess plasma cells containing Russell bodies. Four of the patients suffered from multiple myeloma, whereas the fifth probably had a different disease or was in a premyeloma stage. The Russell bodies stained blue with the May-Grünwald-Giemsa stain and were found both in the nuclei and in the cytoplasm of the plasma cells. Ultrastructural studies showed that the Russell bodies were osmophilic and those located in the cytoplasm were always situated within the cisternae of the rough endoplasmic reticulum. The intranuclear Russell bodies were always surrounded by a triple layered membrane, and some evidence was obtained that these bodies were first formed within the perinuclear space of the cells. Immunofluorescence studies using anti-L chain conjugates showed a positive marginate straining of the intranuclear as well as the cytoplasmic Russell bodies of the cells from all patients. Only one patient had cells with Russell bodies which also stained their location in the plasma cells. It is concluded that some plasma cells in multiple myeloma may produce an excessive amount of L chains which, in combination with a failure in the secretion of immunoglobulin molecules, may lead to the formation of Russell bodies.