The resistance transfer factor (RTF) episome was studied by measuring its inactivation by Co60 gamma radiation, by incorporated P32, and by tritium incorporated as tritium-labeled thymine. The D37 for Co60 irradiation was 7 to 9 × 104 rad. Growth of the bacteria harboring the RTF in BUdR (bromouracil deoxyriboside) increased the sensitivity of the RTF to the gamma radiation. The RTF was markedly inactivated by tritium after growth of the host (thymine requiring) bacteria in tritium-labeled thymine, thus further establishing the presence of thymine in the genome of the RTF. Assuming the efficiency of inactivation by P32 to be 10%, the phosphorus content of the RTF was estimated to be about 2 × 105 P atoms/episome. The data suggest the RTF contains double stranded DNA with a molecular weight of the order of 3 to 8 × 107.