The precise distribution of DNA and RNA within the human Sertoli cell nucleolus has been investigated, at the ultrastructural level, by cytochemical and molecular immunocytochemical techniques. In Sertoli cells, the nucleolar components show a typical spatial distribution. The fibrillar centres are not surrounded by a layer of dense fibrillar component, but come in contact only with strands of dense fibrillar component. These fibrillar parts of strands are the extensions of granular strands connected to a large granular mass. These strands delimit numerous nucleolar interstices in which chromatin fibres are clearly obvious. Using the in situ terminal deoxynucleotidyl transferase/immunogold procedure for detecting DNA, we find evident label exclusively over the chromatin fibres enclosed in the nucleolar interstices and over the fibrillar centres, and no significant label over the dense fibrillar component and granular component of the nucleolus. Furthermore, using the polyadenylate nucleotidyl transferase/immunogold procedure for detecting RNA, we show that label is deposited not only over the granular component and dense fibrillar component, as expected, but also quite obviously over the fibrillar centres. No label is seen over the interstices containing chromatin.