Wildtype 10T1/2 cells and 42 clonally derived morphologically transformed 10T1/2 cell lines were evaluated for expression of p53 mRNA and protein. Wildtype 10T1/2 cells expressed detectable levels of p53 mRNA and multiple immunologically distinct forms of the p53 protein, including both PAb240- and PAb246-reactive conformations of the protein. However, DNA sequence analysis revealed no mutations within the coding sequence of the expressed p53 gene(s) in wildtype 10T1/2 cells. Morphologic transformation of 10T1/2 cells resulted in clonal cell lines that overexpressed p53 mRNA and total p53 protein (PAb421-reactive) relative to the level of expression in wildtype 10T1/2 cells. Immunoprecipitation analysis demonstrated the expression of both PAb246-reactive and PAb240-reactive conformations of the p53 protein in all morphologically transformed 10T1/2 cell lines evaluated, irrespective of the tumorigenic potential of the individual line. Nonetheless, expression of PAb240-reactive p53 protein at levels equal to or higher than that found in wildtype 10T1/2 cells was associated with tumorigenicity in morphologically transformed 10T1/2 cell lines, suggesting that the relative contribution of PAb240-reactive p53 protein to the total cellular p53 protein represents an important determinant for the expression of the tumorigenic phenotype. Our results demonstrate that altered expression of p53 represents a common feature of morphologically transformed 10T1/2 cell line, suggesting that alteration of the expression of the p53 tumor suppressor gene may represent an early and necessary step in the morphologic transformation of 10T1/2 cells. Additionally, overexpression of PAb240-reactive p53 protein may influence the tumorigenic potential of morphologically transformed 10T1/2 cell lines.