1. It has been shown that the ash of liver, hemoglobin, and glutathione each exerts a very slight beneficial effect on the growth of sarcomatous fibroblasts of the rat, or on the condition of their cells when cultivated in a synthetic medium. 2. The addition of all three of these substances, or of only glutathione and hemoglobin, to a mixture of casein digest, glycocoll, and nucleic acid gives a medium in which sarcomatous fibroblasts of the rat proliferate for a considerable time as rapidly as in embryo juice. 3. The mixture is not as adequate a nutritive medium as embryo juice, for after a time dead cells are found surrounding the central fragment of the culture, and after several passages the growth becomes thinner. 4. The hypothesis is suggested that glutathione and hemoglobin may function not only by regulating the respiration and oxidation-reduction reactions within the cell, but also by regulating the oxidation-reduction potential of the medium. 5. It is suggested that the failure to obtain growth of fibroblasts in mixtures of amino acids or of the products of complete acid hydrolysis of proteins is in part due to the absence of glutathione, and that this substance is not synthetized by fibroblasts. 6. The growth of normal fibroblasts of the rat is also increased by the addition of the above mentioned substances to a synthetic medium.