A mutation in the tumor suppressor p53 gene resulting in an Arg-->Ser substitution in position 249 is found frequently in human hepatocellular carcinomas associated with hepatitis B infection and with aflatoxin exposure. To determine the significance of this mutation in an in vivo experimental model using transgenic mice, we introduced a two-nucleotide change in the mouse p53 gene at amino-acid position 246, which is equivalent to position 249 in human p53, by the recombinant polymerase chain reaction mismatched primer method. This p53 mutation resulted in the same change, an Arg-->Ser substitution, as in the human p53 gene at position 249. We now report that the protein product of this mutant mouse p53ser246 had properties similar to those of the wild-type protein when tested by binding to (i) monoclonal antibodies PAb246 and PAb240, ii) simian virus 40 large T antigen, and (iii) heat-shock protein. However, it had mutant-type transforming properties when tested for colony formation with an osteosarcoma cell line. It was not active, as is wild-type p53, in transcription activation of the muscle creatine kinase promoter. These properties are the same as those found in the p53trp248 product of the p53 mutation associated with the Li-Fraumeni syndrome. Although less is known about the human p53ser249 product associated with hepatocellular carcinoma, the mutant murine p53ser246 protein shares the known properties of the human gene product.