A method is described for using apical sections of Avena coleoptiles grown in the dark for 70 hours as an assay for IAA and for potential precursors of IAA. The experiments were carried out under as sterile conditions as possible.Tryptophan appeared not to act as a precursor of IAA. Tryptamine however showed a marked stimulation of growth indicating its possible conversion to IAA. Other compounds that stimulated growth and hence may be regarded as potential precursors of IAA were anthranilic acid and indole. Kynurenine and shikimic acid were among those without an effect. A hypothetical route for the biogenisis of IAA is suggested based on the findings of this work.