Heating of Chinese hamster fibroblasts (46 degrees C, 10 min) results in sharp inhibition of protein biosynthesis in the homogenate, nuclei and, in a lesser degree, in the nuclear matrix. The ratio of specific radioactivity of nuclear matrix 35S-proteins to the homogenate radioactivity taken for 100% increases after the heat shock 2,5-fold. Thus, protein biosynthesis in the nuclear matrix is more stable to the damaging action of heat shock than that in the homogenate and nuclei. The nuclear matrix was shown to contain heat shock proteins with Mr of 82-84, 70 and 26 kD, the 70 kD polypeptide being predominant. This polypeptide revealed in 4 hours is intensively accumulated at the 6th hour and remains unchanged by the 17-24th hours after cell heating. The 70 kD heat shock polypeptide can be separated by two-dimensional electrophoresis into two subfractions differing in terms of pI from other proteins revealed within this time interval in the unheated cells.