The simplest guide to cell proliferation that can be obtained by the use of flow cytometry is the S-phase fraction (SPF) calculated from DNA histograms. Measurement of such histograms was one of the earliest applications of flow cytometry, being first reported in the late 1960s. SPF is the fraction of cells in the S phase of the cell cycle and is broadly equivalent to a tritiated thymidine labeling index ([3 H]TdR LI). The advantages of SPF as a proliferative index include that it can be obtained rapidly from fresh, frozen, or paraffin wax-embedded tissue without the need for radioactive chemicals. The disadvantages include the need to disaggregate solid tissues, thus losing tissue morphology, and the fact that, like mitotic index and [3 H]TdR LI, SPF is only a crude proliferative index, which gives no details of the rate of cell proliferation. Flow cytometry offers a wide range of more sophisticated methods that allow more detailed analysis of the cell cycle and the rate of cell proliferation, and one such method involving bromodeoxyuridine (BrdUrd) labeling is described in this chapter. In the section on further reading, reference is made to sources that describe the combined measurement of DNA content and cell cycle related proteins (1 ,2 ). Also beyond the scope of this chapter are applications of flow cytometry that allow the measurement of cell death and differentiation, but references to these areas are also included (3 ). The literature on DNA flow cytometric studies is enormous, and a considerable number of such studies have looked at various aspects of the relationship between proliferation and metastasis.