Cancer therapy drugs, such as diamminedichloroplatinum (cisplatin), mitomycin C, etoposide and a number of other compounds, as well as energy-rich radiation, are known to act on cellular DNA. These agents are shown to induce nuclear accumulation of the so-called tumor-suppressor protein p53 in fibroblastoid cells, as well as in epithelioid normal and immortalized cells of murine, simian, and human origin. p53 accumulation starts a few hours after treatment and can remain detectable in surviving cells for at least 20 days. Accumulation occurs because of increased p53 protein stability and depends on ongoing translation. It is not the result of enhanced gene expression. A number of cell cycle inhibitors do not affect p53 protein accumulation, suggesting that the process may start from several points in the cell cycle. Since the increase in the nuclear p53 protein levels occurs within a few hours in most of the treated normal diploid cells, it is unlikely that the accumulated p53 protein is derived from a mutated p53 gene. The results obtained are in accordance with the view that the DNA damage-induced p53 accumulation may either inhibit cell growth, allowing DNA repair processes, or, in the case of severe damage, initiate apoptosis.