The mechanism by which simian virus 40 converts the abortive adenovirus type 7 infection of monkey cells into an efficient lytic infection has been investigated. Analysis of ribonucleic acid (RNA) synthesis during unenhanced and enhanced infection of monkey cells has shown that adenovirus RNA synthesized in the abortive infection contains both “early” and “late” sequences. In hybridization competition experiments, early adenovirus RNA from human cells prevented the hybridization of only 20% of the adenovirus RNA transcribed in unenhanced infection. Further, the RNA from unenhanced cells was able to completely block the hybridization of RNA synthesized during enhanced infection. Finally, virus-associated RNA, which is a late RNA transcribed in lytic adenovirus infection, is also produced in the unenhanced infection. An accompanying paper describes a marked deficiency in adenoviral capsid protein synthesis in the unenhanced infection. We conclude that RNA sequences, which are sufficient to code for the synthesis and assembly of structural proteins of adenovirus, are transcribed but are not efficiently translated in the unenhanced adenovirus infection of monkey cells.