A major nucleolar protein with a molecular weight of 100 kD is directly implicated in the transcription of pre-ribosomal RNA (pre-rRNA) and appears to be cleaved into specific maturation products during pre-ribosome biogenesis. Polyclonal antibodies which recognize the 100 kD protein and its products were used to determine the correlation between rDNA transcription and these proteins. Actinomycin D (AMD) was used to block selectively rDNA transcription (AMD 0.1 microgram/ml). Immunoperoxidase and immunogold staining were carried out in untreated and treated cells. Digitalization allowed the quantification of label according to the nucleolar components and the cellular areas. In exponentially growing cells, the dense fibrillar component was shown to contain more 100 kD protein than the granular RNP component but both nucleolar components were positively immunostained. The distribution of the 100 kD protein was rapidly modified by AMD: loss of label occurred first in the dense fibrillar zone of the nucleolus, demonstrating the correlation between rDNA transcription and the presence of this protein. However, one part of the protein remains in the segregated nucleolus after 1 h of AMD treatment, thus supporting the structural function of this protein.