STRUCTURE OF DOLICHOL Dolichol is a polyisoprenoid substance and, con-sequently, is among the largest lipids in the cell (Fig. 1). This polyunsaturated compound has an a-saturated isoprene residue, which may either be in the form of a free alcohol, phosphorylated or esterified with a fatty acid [3-6]. In animal cells the distribution of the various poly-isoprenoids shows a distinct pattern (Table 1). The a-unsaturated form (polyprenol) occurs in small amounts (-1% of the total) under normal conditions [7-10]. When dolichol synthesis is disturbed, such as in human liver cancer cells, the amount of this unsaturated lipid may increase considerably to 10-30% of the total (I. Eggens et al., unpublished work). The amounts of polyprenyl phosphate and pyrophosphate are also very low. The latter compound, with different isoprene lengths, should be present in tissues as intermediates in dolichol biosynthesis, but the distribution of these derivatives has not yet been studied. Short polyisoprenes with less than 15 residues are not present in animal cells, and those with11 residues have