The dynamic organization of nucleoli requires new tools for quantification of nucleolar fluorescence. The nucleolar accumulation of B23 and fibrillarin, frequently used as marker proteins for the nucleolus, is sensitive to changes in growth conditions. HeLa cells were grown under normal conditions or exposed to a 1-hour heat shock at 45.5°C. Cells were fixed and processed for indirect immunofluorescence. Size bar is 20 μm. Note that a portion of either protein relocates from the nucleolus upon heat stress.