An extensive series of histological sections reflecting the various states of normal breast tissue, and a range of benign and malignant lesions, were examined for the expression of the p53 protein using a panel of anti-p53 antibodies. In 2 separate series the results of using frozen or methacarn-fixed, paraffin-embedded sections were compared. Strong positive staining for p53 was detected in over 50% of the malignant lesions when frozen sections were used. This number fell to just over 20% when methacarn-fixed sections were examined. In neither series was any p53 staining seen in normal breast or in the benign lesions. Studies by Western blotting on breast cell lines confirmed that this histological signal is due to a pronounced over-expression of the p53 protein. Earlier studies show that this over-expression is associated with mutation of the p53 gene. Mutation of the p53 gene with over-expression of the mutant protein is therefore one of the most frequent specific genetic changes in malignant breast cancer.