A tumour suppressor function for p53 is indicated in human lung cancer and in carcinoma of the colorectum. Loss of suppressor function, by mutation of the p53 gene, is associated with activation of p53 as an oncogene. The suppressor (wild type) and oncogenic (mutant) forms of the murine p53 protein are distinguishable at the molecular level by reactivity with anti-p53 monoclonal antibodies. For example, activated mutant p53 fails to react with PAb246 (p53-246 degrees). We now demonstrate that wild type p53 mRNA can be expressed either as p53-246+ or p53-246 degrees. We propose that p53-246 degrees may represent an allosteric variant of wild type p53 compatible with positive growth control. Thus, for wild type p53 the variants p53-246+ and p53-246 degrees may reflect suppressor and activator functions of p53 in the normal control of cell proliferation. For human p53 we present evidence that the epitope recognised by PAb1620 is analogous to that for PAb246 on murine p53. Thus the epitope for PAb1620 may prove to be of use as a marker for wild type human p53 with anti-oncogenic function.