It has been reported that nuclei from HeLa cells are responsible for approximately 10%-15% of total cellular protein synthesis. We show here that isolated Chinese hamster ovary (CHO) and HeLa cell nuclei are essentially inactive for translation, and that the earlier results were most likely due to cytoplasmic contamination. Moreover, we suggest that the nascent polypeptides observed in nuclei of permeabilized cells may have been due to "overpermeabilization" and consequent damage to the cells. Based on this information, we conclude that nuclear protein synthesis, if it exists, is limited to less than 1% of that in cells.