1. A non-dialyzable fraction from fresh bakers' yeast stimulates the formation of S. muscae virus in cells in synthetic medium in the log phase of multiplication.
2. A similar fraction was not found in calf thymus, pancreas, or liver.
3. The active substance in this fraction has been partially purified.
4. This substance is taken up by the cells. In the absence of virus the added substance is metabolized to a form no longer available for virus formation.
5. A purified yeast fraction, which stimulates adaptive enzyme formation in yeast, has been found to stimulate virus formation in the S. muscae system.
6. The similarities between the yeast fraction that stimulates adaptive enzyme formation and the yeast fraction that stimulates virus formation are discussed.