Mutations in the p53 tumor suppressor gene are the most commonly observed genetic alterations in human cancer. The majority of these mutations occur in the conserved central portion of the gene, but there has been little information about the function of this region. Using proteolytic digestion of the 393-amino-acid human p53 protein, we have identified a 191-amino-acid protease-resistant fragment (residues 102-292) that corresponds to the central portion of p53, and we show that this core fragment is the sequence-specific DNA-binding domain of the protein. DNA binding is inhibited by metal chelating agents, and we find that the core domain contains zinc. Proteolytic digests also reveal a 53-amino-acid carboxy-terminal domain which we show to be the tetramerization domain of p53.