Loss of function of the tumor-suppressor protein p53 is, in general, either caused by mutation, inducing a conformational change, or by binding to inactivating cellular (e.g. MDM2) or viral (e.g. SV40 large T) proteins. In adenovirus type 12 (Ad12)-transformed cells, p53 is stabilized without detectable binding to the Ad12 E1B/54 kDa protein and still present in a wild-type conformation but contains a mutant-like activity in cellular transformation. In this study we examined whether the changed characteristics of p53 in Ad12-transformed cells are correlated with changes in phosphorylation or complex formation of the protein. By making tryptic phosphopeptide maps we found a significant increase in the phosphorylation of the N-terminus of p53. Furthermore, expression of E1A was found to be essential for the altered phosphorylation, while expression of only Ad12 E1B/54 kDa is sufficient to increase the protein half-life. Additionally, we observed p53 to be present in increased molecular weight complexes in Ad12-transformed cells. We conclude that both the phosphorylation and oligomerization of p53 is changed as a result of Ad12 transformation.