Formation of hybrids between viral deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) was used to detect virus-specific RNA in the nuclei and polyribosomes of transformed and tumor cells induced by "highly" oncogenic human adenovirus (Ad) types 12, 18, and 31. The presence of virus-specific RNA in the cell nucleus, and the inhibitory effect of actinomycin D on its synthesis, suggest that adenovirus-specific RNA is transcribed from a DNA template in the nucleus. Ad 12, 18, and 31 virus-specific RNA did not hybridize significantly with the DNA of the "weakly" oncogenic adenovirus group (Ad 3, 7, 11, 14, 16, and 21) or with that of nononcogenic Ad 2 and 4. Labeled RNA from Ad 12, 18, and 31 tumor cells hybridized with heterologous Ad 12, 18, and 31 DNA 30 to 60% as efficiently as with homologous DNA. Thus, common viral genes are transcribed in tumor cells induced by Ad 12, 18, and 31.