Loss of or mutations in p53 protein have been shown to decrease both radio- and chemosensitivity. The present study assessed the p53 gene status, ability to arrest in G1 of the cell cycle, the functionality of the p53 transduction pathway, and apoptosis following treatment with radiation in a series of drug-resistant human breast cancer cells to determine whether p53 alterations occur during the development of drug resistance. We used 13 sublines derived from MCF-7, ZR75B, and T47D cells, which were resistant to doxorubicin, paclitaxel, vinblastine, cisplatin, etoposide, and amsacrine. Eleven of 12 drug-resistant sublines retained the parental p53 gene status, as determined by sequence analysis and functional yeast assay; only one subline was found to have acquired a mutation in the p53 gene. The MCF-7 TH subline was found to both acquire mutated p53 and to have major changes in p53 protein expression and function. In 12 other drug-resistant sublines, the G1 checkpoint was conserved or only slightly impaired. A normal accumulation of p53, p21Cip1/Waf1, and Mdm2 proteins and hypophosphorylation of Rb protein occurred in response to radiation with only small differences noted in the kinetics of p53 and p21Cip1/Waf1 induction. Increased susceptibility to apoptosis was found in the ZR75B drug-resistant sublines, whereas no evidence for apoptosis was observed in the ZR75B, MCF-7, and T47D parentals and the MCF-7 and T47D drug-resistant sublines. This effect could not be explained by alterations in bcl-2 or bax expression. Our results demonstrate that alterations in: (a) p53 gene status; (b) ability to arrest in G1; (c) induction of p53 protein and p53-dependent genes; and (d) decreased activation of apoptosis is not a requirement for the onset of drug resistance. The function of p53 appears to be dissociated from drug resistance in our model system.