A prerequisite for using flow cytometry (FCM) is the availahility of isolated single eells. Proeedures for separation and isolation of keratinocytes from animals and man are available, and the resulting single cell suspensions have been subjeeted to FCM measurements. The major advantage of the method is the aecuracy and speed with whieh a variety of eellular eonstituents ean be quantified. FCM of keratinocytes has, hitherto, been mainly eonfined to measurements of nuelear DNA for estimation of cell-eyele distributions and lor ploidy studies. In mouse epidermis, eell-cycle distributions were estimated from sequentially obtained DNA histograms and evaluated with other cell kinetie measurements, resulting in new information about epidermal cell-cycle progression, not achievable by any of the methods alone. The best way, therefore, to inerease our knowledge of keratinoeyte proliferation, is the combined use of DNA FCM and other cell kinetic methods. DNA FCM has also been applied to healthy and diseased human epidermis, and may add valuable information to the classification of skin disease in selected cases. It is believed that further progress in the characterization of keratinoeyte growth and development will depend on parameters other than DNA alone.