Transcriptional and post-transcriptional regulation of proliferation-associated nucleolar P120 protein expression was examined in 1 normal and 5 malignant breast cell lines. The 6 breast cell lines could be placed into 3 categories on the basis of in vitro growth rate. BT549 and HBL100 grew rapidly; MCF-7/6, MCF-7/AZ and Hs578T grew at a moderate rate; and Hs578N normal epithelia grew slowly. There was a significant correlation between the growth rate, measured by percentage of S-phase fraction of cells or doubling time of cell numbers and the steady-state levels of either P120 protein or P120 mRNA. Next, the mechanisms responsible for the increased level of P120 expression, which is associated with rapidly growing breast carcinoma, were examined. The stability of P120 mRNA was measured by densitometry of quantitative Northern blots of mRNAs from cells treated with actinomycin D. Before the expected decay, a sharp increase in P120 mRNA level was detected shortly after inhibiting the overall RNA or protein synthesis. The calculated half-life of P120 mRNA was very similar (1.8 ± 0.2 hr) in all 6 cell lines examined. The transcription rate of the P120 gene was determined by densitometric analysis of quantitative nuclear run-off assays. A significant positive correlation was found between the transcription rate of the P120 gene and the steady-state levels of either P120 protein or P120 mRNA. Our conclusion is that the expression of P120 protein is transcriptionally regulated in these breast cells. Therefore, the characteristically high level of P120 protein and mRNA found in most breast carcinomas is due to the altered transcription rate of the P120 gene in transformed cells. © 1995 Wiley-Liss, Inc.