The availability of antibodies which recognise p53 protein in paraffin-embedded tissue has created the opportunity to use immunohistochemistry to study the expression of p53 in a wide variety of clinical material. In this paper we have investigated the relationship between the type of fixative and the pattern of p53 staining in mammary carcinoma. Optimal results were obtained from breast tissue fixed in phenol formol saline, methacarn or cold formol saline with positive staining for stabilised p53 protein occurring in 69/95 (73%) cases studied. Care must be taken in the interpretation of these results since positive staining for p53 protein is not always indicative of mutation of the p53 gene. Furthermore, a range of staining patterns is seen in mammary carcinomas, making interpretation difficult. Assessment of staining needs to be standardised in order that different studies can be compared. However, in breast carcinoma, p53 immunohistochemistry appears to give information relating to tumour grade and, independently, to prognosis.