A nonhistone chromosomal protein, high mobility group (HMG) 1, is ubiquitous in higher eukaryotic cells and binds preferentially to cisplatin-modified DNA. HMG1 also functions as a coactivator of p53, a tumor suppressor protein. We investigated physical interactions between HMG1 and p53 and the influence of p53 on the ability of HMG1 to recognize damaged DNA. Using immunochemical coprecipitation, we observed binding of HMG1 and p53. Interaction between HMG1 and p53 required the HMG A box of HMG1 and amino acids 363-376 of p53. Cisplatin-modified DNA binding by HMG1 was significantly enhanced by p53. An HMG1-specific antibody that recognized the A box of this protein also stimulated cisplatin-modified DNA binding. These data suggest that an interaction with either p53 or antibody may induce conformational change in the HMG1 A box that optimizes DNA binding by HMG1. Interaction of p53 with HMG1 after DNA damage may promote activation of specific HMG1 binding to damaged DNA in vivo and provide a molecular link between DNA damage and p53-mediated DNA repair.