Serum P419 from a patient with rheumatoid arthritis with a high specificity immunolabeling nucleoli in various mammalian cells has been identified. On the Western blots of total cellular proteins or proteins extracted from isolated nucleoli it cross-reacted with a doublet of polypeptides of 97 and 94 kDa. That is why this serum has been concluded to recognize UBF, or RNA polymerase I-specific transcription initiation factor. It was shown that UBF remained bound to the nucleoli or nucleolus organizing regions (NORs) of mitotic chromosomes despite the level of rDNA transcription. Nevertheless, intranucleolar localization of UBF was dramatically changed after partial or complete block of rRNA synthesis. In pycnotic cells positive labeling was found within the whole nucleus and cytoplasm instead of nucleolus. In metaphase UBF molecules are unequally distributed between the particular NORs, whereas in anaphase they are uniformly allocated between the daughter cells.