The thymidine analog, 2,3-dideoxythymidine (ddT), is rapidly phosphorylated and incorporated terminally at 3-ends of growing DNA chains in replicating mammalian cells. Following some initial loss of ddT incorporated into DNA chains, the major portion is retained for periods equivalent to more than two normal cell generations. Some ddT appears at the termini of oligonucleotides, a portion of which have chromatographic properties suggesting internally complementary sequences. While these oligonucleotides may include degraduation fragments, it is possible that some represent replication initiation sequences.