Lymph node cells of rabbits injected with sheep erythrocytes, identified as antibody-producing by their ability to produce plaques of hemolysis in erythrocyte-containing agar layers, have been examined by electron microscopy, by the use of a procedure devised for subjecting single cells to such examination. The antibody-producing cells thus examined were found to fall into two classes, according to the current terminology: some were in the category of lymphocytes, and others, in the category of plasma cells. Within each class, cells were found to vary in certain characteristics, especially in the degree of development of such organelles as the nucleolus, Golgi apparatus, and the endoplasmic reticulum. In the case of the endoplasmic reticulum especially, it could be seen that a series of these plaque-producing cells, ranked in order of increasing size and development of the endoplasmic reticulum, would extend over a considerable range from those lymphocytes with the least developed organelles to the mature plasma cells with the greatest development of these structures.