Polyamines intensify the effect of a heat shock on Escherichia coli M-17 cells. The lethal effect of polyamines rises in the spermidine--spermine series as their concentration and the duration of heat action is increased. The effect of polyamines on the cells subjected to a heat shock is not associated with the activity of amine oxidases and, apparently, does not destabilize the membranes. As was demonstrated using electron microscopy, the cells undergo morphological changes in the presence of polyamines in the course of a heat shock; in particular, electron-dense regions appear in the nucleoid zone, presumably, due to DNA conformational rearrangements during the shock in the presence of polyamines.