Transformed and nontransformed cells differ in stability and cell cycle regulation of a binding activity to the murine thymidine kinase promoter. A DNA binding activity to an upstream region of the murine thymidine kinase gene is regulated differently in a transformed and nontransformed cell line pair. Differences in regulation were observed (i) after serum levels were reduced, (ii) when serum levels were returned to initial high levels, and (iii) while protein synthesis was inhibited. After reduction of serum levels, the binding activity was unstable in nontransformed BALB/c 3T3 clone A31 cells but was significantly more stable in benzo[a]pyrene-transformed BALB/c 3T3 cells. After serum concentration was returned to high levels, the kinetic pattern of the binding activity differed between nontransformed and transformed cells. While protein synthesis was inhibited, the binding activity was unstable in nontransformed cells and stable in transformed cells. Partial inhibition of protein synthesis--a more stringent condition to test instability--prevented the induction of the binding activity in nontransformed cells. Previously, the labile protein hypothesis set forth the criterion that a protein regulating the onset of DNA synthesis should be unstable in nontransformed cells and stable in transformed cells. The DNA binding activity described here satisfies this criterion. 