EM investigation of Ag-AS-NOR staining after short glutaraldehyde prefixation followed by Carnoy fixation maintained good ultrastructural preservation and reactive selectivity. This enables exact localization of silver deposits both in the fibrillar centers of typical or segregated nucleoli during interphase, and in chromosome NORs during mitosis. These results argue in favour of the possibility that fibrillar centers are the interphasic counterpart of chromosome NORs. Special structures such as nucleolar blobs and remnants usually considered to be of nucleolar origin, were also stained. - These findings seem to indicate a relationship between the distribution of the silver-stained proteins, the arrangement of the nucleolar structures and the degree of nucleolar activity resulting from the experimental conditions. These results are of interest at the time when the concept of the nucleolar matrix is gradually emerging.

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The localization of antibodies in cells of the plasmacytic series was established at the light microscope level, first indirectly by comparative immunological and histological studies (ll) and then directly by the binding of cellular antibodies with microscopically visible antigens (bacteria) (22) and by immunofluorescent (8, 9) and radioautographic (3) techniques. Since these early studies, there has been an impressive number of investigations of plasma cells (see reviews by Nossal, reference 18, and Feldman, reference 12) which have confirmed and extended the original observations. More recently, de Petris et al. (i0) revealed the ultrastructural site of antibody in plasma cells with a direct immunoferritin technique. They used ferfitin as an antigen to immunize rabbits, then exposed formaldehyde-preserved lymph node ceils to ferritin and obtained specific antigen-antibody precipitates. The distinctive structure of ferritin permitted the localization of this antigen-antibody complex in the cisternae of the endoplasmic reticulum and the perinudear space in young plasma cells.Enzymes have recently been used as antigen to immunize rabbits and to trace the distribution of anti-enzyme antibodies in differentiating plasrua cells at the fight microscope level (2). In this study, we have used this approach to localize antibody at the ultrastructural level. Horseradish peroxidase was employed as an antigen to hyperimmunize rabbits and to trace the distribution of the antibody to peroxidase in differentiating plasrua cells, from the earliest appearance of antibody in heruocytoblasts 2 days after the booster injection of antigen through its accuruulation in wall-developed plasma ceUs 3-5 days after injection. Like ferritin, horseradish peroxidase is a good antigen as judged by the high titers of precipitating antibodies which are obtained. As in the ferritin technique of de Petris et al. (10), this is a direct method which precludes the necessity of conjugating the antigen with an dectron-opaque label and, therefore, penetration of antibody-containing cells by the antigen is more readily attained. The site of bound antigen is then revealed by the cytocheruical pro-*

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