Horizontal starch gel electrophoresis of crystalline yeast alcohol dehydrogenase, which had been predialyzed against pH 7.5 phosphate buffer, revealed the presence of eighteen components. Five components possessed dehydrogenase activity. Freeing the system from metal ions resulted in a reduction of the number of active components to two and predialysis of the enzyme against Tris/citrate instead of phosphate buffer resulted in a single active component. Pretreatment of the enzyme with 1,10-phenanthroline in pH 7.5 phosphate buffer but not Tris/citrate buffer resulted in the formation of a new component which migrated more rapidly than the active band. The usefulness of starch gel electrophoresis as a criterion for the homogeneity of proteins is discussed in the light of these findings.