Exposure of cells to hyperthermic temperatures (43° to 48°C) results in an increase of the total protein mass which coisolates with the nuclei. Prominent physiological effect of heat-shock on nuclear structure is inhibition of DNA replication and repair, and also RNA synthesis and processing. Heat-shock-induced p rotein mass increase has been shown to correlate with ultrastructural changes and polypeptide composition of nuclear skeleton - nuclear matrix.