Investigation of the events of the cell cycle can be conducted in a number of ways. One method, that of using cell populations which are in synchrony, is currently being exploited because it overcomes the insensitivity of our biochemical methods of analysis in comparison to the minuteness of the individual cell. Apologias for the technique as applied to eucaryotic cells have been clearly stated by Zeuthen in the Introduction to his book "Synchrony in Cell Division and Growth" (1), and it has been dramatically pursued by his group and others using protozoans, particularly Tetrahymena pyriformis. Animal cells are not, perhaps, as readily handled as protozoans, but nevertheless recent work has produced valuable information on the morphological and macromolecular occurrences in the cell as it passes from division to division, and considerable promise has been given for the early explanation of some of the controlling mechanisms of cell growth and differentiation. Earlier articles containing limited references to synchrony in animal cells include Zeuthen's book, two reports of conferences held at Oak Ridge in 1964 (2) and 1967 (3) which have been compiled and expanded into books by Cameron and Padilla (4), and Padilla, Cameron and Whitson (5). A third conference has been held at Wrightsville in 1969. A review article by Scherbaum (6) surveying work with microorganisms contains a number of concepts which are still relevant and a more recent compilation by Neikovid provides a very useful discussion of the effects of various antibiotics on the phases of intermitosis (7). A number of authors have pointed out a distinction between synchronous and synchronized cells (6, 8, 9) and the present article will attempt to maintain this distinction between synchronized cells, where synchrony has been induced by intervention in an entire population of exponentially growing cells, and synchronous cells, i dicating the more passive technique of selecting from the cell population a fraction in which all of the selected cells are at the same stage of the cell cycle. Not all systems, however, can be readily categorzed in either of these ways, nor is it yet clear whether there is a fundamental dif-