We investigated in great detail the fine spatial distribution of nucleic acids within adenovirus-infected HeLa cells by various immunogold labeling procedures. To detect DNA, we used the in situ terminal deoxynucleotidyl transferase-immunogold technique. In addition to the expected evident label over the condensed host chromatin and the structures containing viral double- and single-stranded DNA, label was consistently revealed over round fibrillar spots. By contrast, other virus-induced substructures, such as compact rings, crystalloids, clear amorphous inclusions, and electron-dense amorphous inclusions, displayed no significant label. Except for the viral single-stranded DNA accumulation sites, identical labeling pattern was obtained with the in situ nick-translation-immunogold method. We further labeled the sections with anti-RNA antibodies. Label was present not only over the cytoplasm and the intranuclear fibrillogranular network but also quite obviously over the compact rings and interchromatin granule clusters. None was seen over the other nuclear structures of infected cells, notably over the fibrillar spots. We suggest that these fibrillar spots might be involved in the formation of the viral, non-encapsidated, double-stranded DNA storage site.