Halogenated nucleotides such as the pyrimidine analog bromodeoxyuridine (BrdU) are useful for labeling nascent DNA in living cells and tissues. BrdU becomes incorporated into replicating DNA in place of thymidine and subsequent immunodetection of BrdU using specific monoclonal antibodies allows labeling of cells in S phase of the cell cycle. After pulse-labeling cells or tissues with bromodeoxyuridine, BrdU (Bu20a) Mouse mAb can be used to detect BrdU incorporated into single stranded DNA.