Immunocytochemical detection of cyclins, inhibitors of cyclin-dependent kinases and other cell cycle regulatory proteins, combined with analysis of cell cycle position (DNA content) by multiparameter cytometry offers unique analytical possibilities. This review focuses on applications of the methodology that yield information on the abundance, modification or interactions of these proteins in relation to cell cycle progression that cannot be obtained by other methods. Particular attention is given to the use of multiparameter cytometry in analysis of the mechanism by which antitumor drugs affect cell cycle progression and induce cell cycle phase-specific apoptosis.