SUMMARYPolyporus schweinitziisuffers from a partial deficiency for at least two unidentified growth-promoting substances which are widely distributed in natural products. Both are soluble in organic solvents, adsorbed in charcoal and thermostable. One is“neutral”and less polar; the other acidic and more polar. Concentrates producing a measurable increase in growth at less than 1μg per ml were prepared. The substances may be involved in nitrogen metabolism.