Somatic mutation of the p53 gene is a very frequent event in the development of human neoplasia, and germ line mutations in p53 are responsible for an inherited cancer susceptibility syndrome. Many of the mutations in p53 found in human tumours are point mutations that result in the substitution of a single amino acid in the protein. These point mutant proteins are much more stable than the normal protein and the mutant product accumulates to a high level which permits important information about p53 expression to be obtained by immunochemical analysis. Using bacterial expression systems to produce fragments of human p53 we have isolated and characterized new monoclonal antibodies to p53. These antibodies are suitable for the measurement of p53 in ELISA, immunoblotting and immunoprecipitation analyses. They are especially useful in immunohistochemistry as they are able to react strongly with p53 in conventionally fixed and processed histological sections.