Cytoplasmic extracts of primary rabbit kidney cells inoculated with fibroma virus revealed 2 peaks of DNA complexes (120S and greater than or equal to 410S) in a linear sucrose gradient. Pulse-chase experiments demonstrated a shift in the gradient profile of lighter complexes toward heavier complexes. Synthesis of DNA complexes was inhibited by adding puromycin or actinomycin D. The DNA from virus-infected cultures hybridized 7 to 9 times greater with fibroma virus DNA than did the DNA from noninfected cultuures. The DNA complexes became increasingly resistant to deoxyribonuclease digestion as a function of time during viral growth cycle and produced tumors in rabbits.