Pharmacological activation of wild-type p53 has been found to protect normal cells in culture from cytotoxicity and nuclear aberrations caused by conventional cancer therapeutics. Hence, small-molecule p53 activators could have clinical benefits as chemoprotectants for cancer patients bearing p53-mutant tumors. We have evaluated 16 p53-based cyclotherapy regimes combining p53 activators tenovin-6, leptomycin B, nutlin-3 and low dose actinomycin D, with clinically utilized chemotherapeutic agents (S-and M-phase poisons) vinblastine, vinorelbine, cytosine arabinoside and gemcitabine. All the p53 activators induce reversible cell cycle arrest in primary human fibroblasts and protect them from both S-and M-phase poisons. Furthermore, studies with p53-mutant cancer cell lines show that nutlin-3 and low-dose actinomycin D do not affect the sensitivity of these cells to any of the chemotherapeutics tested. Thus, these two small molecules could be suitable choices for cyclotherapy regimes involving S- or M-phase poisons. In contrast, pre-incubation of p53-mutant cells with tenovin-6 or leptomycin B reduces the efficacy of vinca alkaloids, suggesting that these p53 activators could be effective as chemoprotectants if combined with S-but not M-phase poisons. Discrepancies were observed between the levels of protection detected immediately after treatment and following recovery in fresh medium. This highlights the need to assess both short-and long-term effects when evaluating compounds as potential chemoprotectants for cancer therapy.