Temperature-sensitive mutants of 3T3 cells (H6-15) express the transformed phenotype at 33 degrees C and the normal phenotype at 39 degrees C. Cold-sensitive mutants of Chinese hamster ovary cells (cs4-D3) express the transformed phenotype at 39 degrees C and the normal phenotype, along with a G1 block, at 33 degrees C. When either cell type is under conditions such that it is normal and in a G0 state, the number of S1-sensitive sites in purified DNA, labeled in parental chains only, is zero. When the normal cells are stimulated by 10% serum, the number of S1 sites per 10(5) base pairs increases slightly, to 0.7 in cs4-D3 and 1.1 in H6-15. Under conditions permitting the expression of the transformed phenotype, but not proliferation, the maximum number of S1 sites per 10(5) base pairs is 5 in cs4-D3 and 44 in H6-15. When the stationary transformed cells are stimulated by 10% serum, the number of S1 sites per 10(5) base pairs increases to 6 in cs4-D3 and 43 in H6-15. Furthermore, the DNA from the stimulated transformed H6-15 cells contains at least twice as many S1 sites as the total number of breaks (nicks plus gaps), raising the possibility of the acquisition of stable looped or cruciform structures as the cells are stimulated.