We prepared ultra-thin sections of human myeloma cells, in which the rER was cut tangentially, and studied the make-up and distribution of membrane-bound polysomes electromicroscopically. In IgG myeloma large and small polysomes were detected. The polysome distribution curve showed a high peak at 7 ribosomes and a lower peak at 17-18 ribosomes. IgA-, IgD- and IgE myeloma, as well as macroglobulinemia, showed peaks at 7 and 13 ribosomes. BJP myeloma manifested a sharp peak only at 7 ribosomes. Our results suggest that BJP myeloma has only small polysomes participating in L-chain synthesis, while the other myelomas exhibited large and small polysomes participating in H- and L-chain synthesis, respectively. The quantitative ratio of small and large polysomes was determined on the basis of an analytically corrected direct count.