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Crucial to an understanding of the mechanism of cell replication is a knowledge of the manner in which intracellular and surface membranes develop between successive divisions . The application of zonal centrifugation to separate cells according to their size and hence their relative age (Warmsley and Pasternak, 1970) has enabled us to study the timing of these events by measuring changes in four parameters characteristic of membranes : (a) the cellular content of phospholipid, (b) the activity of enzyme "markers" indicative of mitochondrial and microsomal membranes, (c) the activity of mitochondrial energy-coupling structures revealed by a fluorescent probe, (d) the cell volume . Each parameter was found to approximately double during the intermitotic period of P815Y neoplastic mast cells ; the rate of increase is most rapid during the S period, though clearly distinguishable from that of DNA in that it commences already in G1 and continues into G2 (Warmsley and Pasternak, 1970; Warmsley, Phillips, and Pasternak, 1970) . The fact that P815Y cells contain the H-2 antigen has enabled us to study changes in the surface membrane in rather more detail, by carrying out immune cytolysis (Sanderson, 1964 ; Wigzell, 1965) of cells from different regions of the gradient with the appropriate antiserum . The recent report of cyclic changes in an agglutininbinding site on 3T3 cells as revealed by immune fluorescence studies (Fox, Sheppard, and Burger, 1971) has now prompted us to report on some of our initial findings .

STRUCTURAL ALTERATIONS IN THE SURFACE MEMBRANE DURING THE CELL CYCLE