Certain biological activities of SV40 large T antigen can be attributed to distinct regions on the polypeptide chain of T antigen. Some of these properties are influenced by the interaction of T antigen with cellular proteins. We established a mouse cell line expressing full length T antigen and another cell line expressing a truncated T antigen (T600C) consisting of 600 C-terminal amino acids of authentic T antigen. Morphology, growth properties and actin cable organization indicated that cells expressing full length T antigen were transformed whereas the T600C T antigen expressing cells were non-transformed. Full length T antigen and the T600C fragment of T antigen were both phosphoproteins, both were located in the nucleus and both formed complexes with p53. Rate of synthesis as well as stability of the T600C T antigen and of p53 were reduced in the non-transformed cells when compared to full length T antigen and p53 in transformed cells. Stoichiometry of T antigen and p53 in T-p53 complexes seemed to be different for the T600C T antigen expressing cells when compared to cells expressing full length T antigen. Immunopurified complexes of the T600C T antigen and of full length T antigen with p53 were associated with a protein kinase phosphorylating both proteins. Interestingly uncomplexed full length T antigen was also associated with a protein kinase whereas the T600C T antigen lacked this ability. These data may indicate a correlation between cell transformation and distinct biochemical properties of T antigen and p53 including the association of T antigen with a protein kinase.