We propose to continue our work on the DNA binding properties and the enzymic activities associated with SV40 large T antigen. Previous work has established the following facts with respect to the binding of T antigen to DNA: (1) T antigen binds to double strand DNA is site specific in the sense that it binds with higher affinity to the DNA sequences found at origin of replication in SV40 DNA than it does to random DNA sequences. (2) T antigen binds single strand DNA and this binding also is of a higher affinity than is its binding to random double strand sequences. (3) T antigen will promote the specific unwinding of SV40 DNA sequences. The unwinding of specific SV40 sequences does not require ATP or other energy sources. (4) T antigen possesses an intrinsic ATPase which is markedly stimulated by poly dT molecules having a minimum chain length of 30 to 50 residues of thymidine. We plan to continue our characterization of the ATPase, the DNA binding and unwinding activities of T antigen with the ultimate of understanding the role of T antigen in the initiation of viral DNA synthesis and in cell transformation.