When C(14) carboxyl indoleacetic acid (IAA) is transported through Avena coleoptile sections a fraction of the activity becomes bound. The nature of this bound IAA has been investigated. Upon extraction with solvents and chromatography a substance having the R(F) of IAA in 4 solvents was detected. No evidence could be found for the formation of indoleacetyl conjugates. In pea stem sections subjected to a similar experimental regime good evidence was obtained for the occurrence of conjugates. When IAA was supplied exogenously to coleoptile sections floating in solutions the occurrence of conjugates was shown to be dependent on the presence of the primary leaf. In its absence no conjugates could be detected.On grinding coleoptile sections and subsequent centrifugation at 240 x g the radioactivity was found to be in the tissue fraction as opposed to the supernatant. The radioactivity cannot be removed from the tissue by extraction with water, buffer solution or treatment with ribonuclease. It is readily removed by 10% urea, crystalline trypsin and chymotrypsin. It is therefore concluded that IAA becomes bound to a protein. Bound IAA does not appear to be able to cause growth in Avena coleoptile sections.