The lipid composition of Saccharomyces carlsbergensis 4228 cells grown aerobically in the presence of thiamine and absence of pyridoxine was markedly different from that of cells grown without addition of both of the growth factors. In addition to the previous observations showing a reduction in the levels of unsaturated fatty acids (Nishikawa, Y., Nakamura, I., Kamihara, T. and Fukui, S. (1974) Biochem. Biophys. Res. Commun. 59, 777-780) and lack of zymosterol and ergosterol (Nagai, J., Katsuki, H., Nishikawa, Y., Nakamura, I., Kamihara, T. and Fukui, S. (1974) Biochem. Biophys. Res. Commun. 60, 555-560), the thiamine-grown cells were found to contain low levels of total lipids, sterols (especially in the form of esters), triacylglycerols and total phospholipids. However, relative contents of triacylglycerols and phospholipids to total lipids were higher than those of control cells. Hydrocarbons and diacylglycerols accumulated to appreciable degrees. Phospholipid composition was also influenced by thiamine. The ratio of phosphatidylinositol to total phospholipids increased, whereas that of phosphatidylethanolamine decreased. The levels of phosphatidylcholine plus phosphatidylserine decreased in a similar ratio to that of total phospholipids. It was found that unsaturated fatty acid contents were low in all lipid esters tested. The effect of thiamine was particularly noteworthy in the case of sterol esters. Concomitant addition of pyridoxine with thiamine to the medium brought about a normal lipid composition in the yeast cells.