The nuclear phosphoprotein p53 occurs at elevated levels in many transformed cells. Mutant forms of mouse p53 possess enhanced transforming activity compared with wild type p53. Mutant mouse p53 proteins form complexes with the 70 kDa family of heat shock proteins (HSPs). We previously demonstrated an association between p53 and the 70 kDa HSPs in the human osteosarcoma (HOS) derived cell line HOS-SL. We report here the molecular cloning and sequencing of the p53 gene from HOS-SL cells, and demonstrate that it is in fact mutant. Further, analysis of similar HOS-derived cell lines demonstrates that they also encode the same mutant form of p53, whereas the wild type form of p53 appears to be lost in these cells. Stability studies demonstrate an increased half life of the p53 protein in these cells, in keeping with its association with the HSP 70 proteins. A potential role for this p53 mutant in the transformation process is discussed.