Bromodeoxyuridine (BrdU) is a thymidine analog and is specifically incorporated into DNA during DNA synthesis. Detection of bromodeoxyuridine (BrdU) incorporated into DNA is a powerful tool to study the cytokinetics of normal and neoplastic cells. In vitro or in vivo labeling of tumor cells with the thymidine analogue BrdU and the subsequent detection of incorporated BrdU with specific anti-BrdU monoclonal antibodies is an accurate and comprehensive method to quantitate the degree of DNA-synthesis. BrdU is incorporated into the newly synthezised DNA of the S-phase cells and can thus provide an estimate for the fraction of cells in S-phase. Also dynamic proliferative information (such as the S-phase transit rate and the potential doubling time) can be obtained, by means of bivariate BrdU/DNA flow cytometric analysis.