The synthesis of two hemoproteins, catalase A and catalase T, was studied in mutants of Saccharomyces cerevisiae deficient in heme formation. These mutants can be grown on the end-product heme or on a heme precursor, or on ergosterol and Tween 80 (a source of oleic acid). It was found by immunoprecipitation that, in the presence of heme, catalases A and T were present in the mutants, but that in its absence (growth on ergosterol and Tween 80) the apoproteins of these enzymes were not detectable. In contrast, cytochrome c peroxidase, and some of the subunits of cytochrome c oxidase are present in cells grown without heme (Saltzgaber-Müller, J., and Schatz, G. (1978) J. Biol. Chem. 253, 305-310). Other evidence suggests that absence of catalase T apoprotein under heme-less conditions may be due to control by heme of apoprotein synthesis (G. Ammerer and H. Ruis, unpublished results), rather than increased proteolytic degradation.