The effect of unlabeled cellular DNA upon the reassociation kinetics of labeled double-stranded DNA made by DNA polymerase from avian tumor viruses has been used to measure virus-specific nucleotide sequences in cells. Multiple copies of these sequences were found equally in normal chick cells, in chick cells transformed in culture, and in Rous tumor cells. Copies were also present equally in cells harboring chick helper factor and group-specific antigen, and in cells lacking those characteristics. Quail DNA also contains multiple copies of these sequences, but no copies were detected in HeLa-cell DNA or salmon-sperm DNA.