It has long been known that ergosterol is the main sterol in yeast cells. Recent studies involving the use of gas-liquid chromatography and mass spectrometry have revealed the presence of other sterols in significant quantities in flaccharomyees cerevisiae (LONOLEY et al. 1968, HUNTER and ROSE 1972, BARTON et al. 1972) but the present authors have found that in Kluyveromyces fragilis only ergosterol is detectable (PENMAN and DUFFUS 1974). Because of the relative paucity of knowledge concerning the flux of lipid components during the cell cycle, it seemed of interest to examine the accumulation of this sterol fraction during the cell cycle in K . fragilis.
Diethyl ether was redistilled before use and stored dry. Cyclohexane was obtained from British DrugHouses Ltd.
Stock cultures of Kluyveromyces fragilis NCYC 100 were maintained on 6% OXOID Malt Extract Agar and batch cultures grown up in 2% OXOID Malt Extract Broth (MEB) at 30 "C in an orbital incubator operating a t 160 rpm.
Synchronous cultures were prepared using the selection method of MITCHISON and VINCENT (1966). Where it was desirable to obtain larger numbers of synchronously dividing cells, two or more identical gradients were used and cells removed from exactly the same point in each gradient. After re-inoculation into fresh growth medium, the yeasts were allowed to grow for one hour before sampling, after which time samples were removed at 20 min intervals and cell numbers determined using a haemacytometer (DUFFUS and PENMAN 1973). These samples were frozen