1. There is an ether-extractable substance in the cotyledons and leaves of radish plants that is capable of causing growth inhibition and positive Avena coleoptile curvatures. 2. It is possible to analyze for this substance on a quantitative basis. 3. This substance occurs in other plants as well as radish. 4. With growth of radish leaves their inhibitor content decreases and is finally replaced by auxin. 5. This inhibiting substance has no acidic or basic groups and can readily be hydrolyzed to form a growth promoting substance. 6. Inhibitor has no polarity of movement in the radish or Avena plant. 7. Auxin contaminating the crude inhibitor extract may be removed by use of the "inverse transport" purification method. 8. Inhibitor may be hydrolyzed at cut plant surfaces to form auxin. 9. Inhibitor is transported through Avena coleoptiles at approximately 11 mm. per hour, or at the same rate as auxin. 10. In many tests for physiological activity, inhibitor behaves like auxin. 11. The auxin diffusing from r...