Yeast mutants defective in ergosterol synthesis are valuable tools for investigating sterol metabolism. Both sterol mutants and sterol auxotrophs have been utilized in determining what sterol structural features are required for yeast cell viability. Both types of mutants can also be studied to ascertain how changes in sterol structure affect membrane properties. Other aspects of sterol metabolism, such as the specificity of sterol esterification, have been elucidated by the sterol auxotrophs. In broader applications, interrelationships between sterol metabolism and other cellular functions (e.g., heme metabolism) may also be examined with these mutants. By analyzing the lipid composition of the sterol mutants, on the other hand, much of the ergosterol biosynthetic pathway has been delineated. The unusual sterols of the mutants can also be obtained to develop assays for the enzymes involved in ergosterol synthesis. Thus, by utilizing mutants, the simple eukaryotic system of yeast may be extended to explore the entire field of sterol metabolism and its relationship to cellular physiology.