Approaches are being developed to study proteins specifically associated with the transformation of cells with tumor viruses and chemical carcinogens. The first category of proteins examined was identified on the basis of their binding to the cellular genome; they were thus candidate gene regulating functions expressed specifically in the transformed cells. At least two chemical carcinogen transformed mouse cell clones were positive for 25-30K protein molecules with nucleic acid binding properties which are comparable to those of DNA tumor virus T-antigens. These cellular T-proteins showed species-specific DNA binding properties. The second category of proteins, characterized on the basis of their ability to interact with cytoskeletal proteins, are candidates for the regulators of cell morphology. In murine sarcoma viruses it has been possible to obtain the sarcoma-specific protein kinase which is likely to be involved in viral transformation in this system.