Hybridization to the separated light (L) and heavy (H) strands of adenovirus 2 DNA in 50% formamide at 37 degrees was used to isolate undegraded virus-specific RNA molecules from the polyribosomes of cycloheximide-treated human KB cells early after infection with adenovirus 2. About 20% of polyribosomal RNA labeled with [(3)H]uridine from 4 to 7 hr after infection was virus-specific. Twice as much labeled RNA was homologous to the L strand as to the H strand. Polyacrylamide gel electrophoresis of RNA selected with unfractionated adenovirus DNA resolved a major component of virus-specific RNA in the 19-20 S region of the gel and smaller amounts of viral RNA in two heterogeneous fractions migrating at 15-18 S and 21-26 S. Selection with individual DNA strands showed that the 19-20 S main size class of early mRNA consists of two homogeneous RNA species with slightly different mobilities, the transcripts from the L and H strand having molecular weights of 7.4 x 10(5) and 7.7 x 10(5), respectively. The 15-18 S RNA hybridized with the L strand and the 21-26 S RNA with the H strand.