Abstract: A method for detecting halogenated precursors incorporated into DNA is presented. The method is based on the selective photolysis of DNA by ultraviolet (UV) light at the sites of an incorporated halogenated precursor, such as the thymidine base analogs 5-bromo-2-deoxyuridine (BrdUrd), 5-iodo-2-deoxyuridine (IdUrd), 5-fluoro-2-deoxyuridine (FdUrd), or 5-chloro-2-deoxyuridine (CldUrd). The 3'-hydroxyl termini of the DNA single strand breaks generated during photolysis may be marked directly or indirectly with a fluorescent label. The DNA termini are directly labeled with fluorochrome-conjugated deoxyuridine triphosphate (dUTP) catalyzed by exogenous terminal deoxynucleotidyl transferase or DNA polymerase (nick translation system). The DNA termini are indirectly labeled with either biotin- or digoxygenin-conjugated dUTP; the incorporated biotin or digoxygenin is then detected following binding of fluorochrome-conjugated avidin or anti-digoxygenin antibody, respectively.