Antinucleolar antibodies are of interest in 2 important areas, namely, autoimmune diseases and specific products involved in the "mitogenic cascade." The former have delineated a series of novel nuclear and nucleolar elements including the recently described proliferating cell nuclear antigens (PCNA), some of which are present in the nucleolus only at specific times in the G1-S phase of the cell cycle. Important new proteins such as "fibrillarin" (34 kD/pI 8.5) and a 125 kD nucleotide containing protein have been identified with antinucleolar antibodies. Protein p145 is a nucleolar PCNA that is present in growing and dividing cells but not in normal resting tissues. Antinucleolar antibodies offer powerful tools, not only for identification of specific nucleolar proteins important in the cell cycle, but also for purification of their genes and analysis of mechanisms of gene control that operate the "time windows" of the cell cycle.