The syn- thesis of respiratory pigments by yeast, either as primary or secondary products, is discussed at length. A number of experiments in which the substances studied were identified spectroscopically are given in de-tail. Results indicate the regular occurrence of kopro-porphyrin in yeast cultured in a porphyrin-free medium, as well as in commercial yeast. The authors state that they have established the fact of the primary synthesis of koproporphyrin by yeast cells.