Treatment of a nontumorigenic clone of AKR mouse embryo cells in culture with a variety of polycyclic aromatic hydrocarbons has resulted in the development of derivative clones which are highly tumorigenic and exhibit other characteristics of the transformed phenotype. A 3-methylcholanthrene-transformed derivative clone (clone MCA) has been compared to the parent clone (clone 2B) with respect to the abundance and diversity of polysomal poly(A)-containing mRNA sequences. Hybridization kinetic experiments show that the poly(A)-containing sequences of both clones are organized into indistinguishable abundance classes, and that the vast majority of the sequences are common to both the parent and derivative clones. The levels of two specific messenger RNAs (α- and β-globin mRNA) which characterize highly differentiated mouse erythroid cells were much less than 1 molecule per cell in either cell type. Titration of a balanced complementary DNA probe to AKR murine leukemia virus (AKR-MuLV) 70S RNA with purified polysomal poly(A)-containing RNA from both parent and derivative clones shows that approximately 5000 and 1200 viral 35S RNA equivalents are present in the cytoplasm of growing and resting clone MCA cells, respectively. Rapidly growing clone 2B cells contain less than about 30 viral 35S RNA equivalents per cell. Viral specific sequences therefore correspond to members of the high abundance class of poly(A)-containing RNA sequences in clone MCA cells and to the low abundance class of sequences in clone 2B cells. Within the limits of detection, this large increase in abundance is characteristic only of viral specific RNA sequences. © 1977.