In the white-opaque transition, cells of Candida albicans strain WO-1 switch reversibly and at high frequency between phases which differ both in colony and cellular phenotype. The lipid and sterol contents of the two phases were compared. White cells were higher in lipid and sterol contents in both mid-exponential and stationary phase cultures. In mid-exponential phase cultures, the lipids of white cells accumulated substantial amounts of apolar compounds, including steryl esters, alkyl esters, triacylglycerols, fatty acids, free sterols and mono- and di-glycerides, while opaque cells accumulated nearly equal proportions of apolar and and polar compounds, mainly phosphatidylethanolamines and phosphatidylcholines. In stationary phase cultures, both white and opaque cells had slightly higher proportions of polar lipids. Major differences in the lipid composition between white and opaque cells involved the contents of free sterols and derivatives of sterols. White cells contained higher proportions of free sterols than opaque cells, while opaque cells contained more steryl glycosides and steryl esters (approximately 2.5 times higher). Comparison of the sterols of the white and opaque cells by UV, TLC and GLC showed that a qualitative as well as quantitative difference exists between the two phenotypes. Fatty acid analysis of white and opaque cells showed that C-16 and C-18 fatty acids are the most abundant in both phenotypes. White and opaque cells varied in their fatty acid composition. The former had higher proportions of palmitoleic (16:1) and stearic (18:0) but lower proportions of linoleic (18:2) fatty acids than opaque cells. Analysis of fatty acids of major lipid classes present in both forms showed that fatty acid pattern varied dramatically according to whether the class had been isolated from white or opaque cells. Our results suggest that the lipid composition (particularly sterol and polyunsaturated fatty acids) of the opaque phenotype resembles that of mycelial cultures. Opaque cells showed more resistance to amphotericin B, nystatin, 5-fluorocytosine (flucytosine) and miconazole nitrate than white cells.