The identification and characterisation of the p53 tumour suppressor has relied extensively on the use of immunological reagents. To facilitate further characterisation of the murine p53 protein (Mp53), and its interaction with other proteins, we have characterised the antigenic sites of Mp53 in fine detail. Using an overlapping Mp53 peptide library we report the identification by Pepscan ELISA of the epitopes of nine antibodies. We have also used this technique to determine whether corresponding epitopes were present in a human p53 (Hp53) peptide library. This comparison was extended to include polyclonal sera of mice immunized with either Mp53 or Hp53, to compare the overall range of antigenic sites. The range of antigenic sites identified by polyclonal sera is very similar, although the N-terminus of Mp53 is clearly not an immunodominant region, in contrast to the N-terminus of Hp53. However, the N-terminus of Mp53 is immunogenic in rabbits as demonstrated by the Pepscan ELISA of CM5 serum (a rabbit anti-Mp53 serum used in analysing p53 expression in mice). Since, very few new antigenic sites were identified in either Mp53 or Hp53, new approaches will have to be employed to identify novel immunological reagents against human and murine p53.