Many of the existing cytokinetic techniques are based on the use of radioisotope-labeled DNA precursors (see Chapter 1–Chapter 3 of this volume). Although numerous data obtained by these techniques contributed greatly to our present knowledge of the cell cycle, there are certain disadvantages related to the use of isotopes in general that limit their wider application, especially in the clinic. The drawbacks of autoradiography, which requires long exposure times and cumbersome grain-count analysis, are well known. There are also problems with the quantitative aspect of the autoradiography; especially of tritium detection (e.g., see ref 51). The alternative of autoradiography, liquid scintillation spectroscopy, precludes analysis of individual cells and thus restricts studies on cell heterogeneity. The variability in pools of endogenous nucleotides and changes in the accessibility of the precursor in vivo or in vitro create additional problems when any quantitative data on the rate of DNA replication (progression through S-phase) must be obtained based on radioactivity measurements. The most important limitation, however, especially when tritium-labeled thymidine is used, is the radiobiological effect of the precursor.