Accumulation of p53 protein resulting in levels detectable by immunohistochemistry (IHC) has been proposed as an indicator of mutation of the p53 gene. We have investigated a panel of 23 fresh-frozen breast cancers by IHC (PAb 1801), Southern and Northern blot analysis, and direct sequencing of the mutation hot spot regions (exons 5-8) of the p53 gene. Three tumors (13%) showed an intense nuclear staining in the majority of malignant cells, but only one of these showed a mutation of the p53 gene (codon 237, Arg to His). Furthermore, a mutation (5-bp deletion) was identified in a tumor that showed no p53 immunoreactivity. Our results indicate that accumulation of p53 protein, as detectable by IHC, is not a reliable indicator for p53 gene mutation in human breast cancer.