Numerous membrane glycoproteins of Saccharomyces cerevisiae are posttranslationally modified by the addition of a glycophosphatidylinositol (GPI). These proteins can be detected most easily by metabolic labelling of yeast cells with 3H-myoinositol or 3H-palmitate. This report summarizes what little is known about the identity, biosynthesis and cellular localization of GPI-modified glycoproteins in Saccharomyces cerevisiae as well as what could be learned from the system with respect to the biosynthesis of GPI's in general.