We assessed cytotoxicity of X rays or fission neutrons and the status of the p53 tumor suppressor gene in irradiated and unirradiated actively growing cultures of human breast cancer MCF-7 cells. One parental or wild-type (WT) and the other resistant to adriamycin (ADRR) were studied within the same experiment. We found that, relative to MCF-7 WT cells, MCF-7 ADRR cells exhibited a small but significant resistance to X rays, but not to fission neutrons. Single-strand conformation polymorphism analysis followed by DNA sequencing and immunohistochemical staining with a p53 protein-specific antibody performed on pooled polyclonal or monoclonal populations of MCF-7 WT or ADRR cells confirmed that wild-type cells have two normal copies of the p53 gene. We discovered p53 loss of heterozygosity and a point mutation in the remaining allele of the p53 gene in adriamycin-resistant cells. This mutation is a splice acceptor site change on the upstream border of exon 5 and results in p53 protein overexpression. No new p53 mutations were observed in MCF-7 WT or ADRR cells surviving either X or fission-neutron irradiations. Our results suggest that the mutant p53 allele affects cytotoxic outcomes of DNA damage from X rays but not from neutrons.