p53, a transformation-related cellular-encoded protein, was found to accumulate at high concentration in transformed cell lines. The results presented here show that p53 biosynthesis is also increased in most induced and spontaneous mouse tumors. Judged by the identity in antigenic determinants (estimated by binding to monoclonal antibodies), size, and partial peptide mapping, I conclude that the p53 molecule found in primary tumors is indistinguishable from that in established cell lines. The fact that p53 is found in heterogeneous populations of primary tumors makes it a convenient biochemical diagnostic marker for the detection of primary tumors in mice. It is found in primary tumors as a phosphoprotein, just as it was found previously in established cell lines. On the other hand, the p53 found at low concentration in normal thymocytes is labeled with [35S]methionine but cannot be found in its phosphorylated form.