We show that the expression of an indole-3-acetic acid (IAA)-modified protein from bean seed, IAP1, is correlated to the developmental period of rapid growth during seed development. Moreover, this protein undergoes rapid degradation during germination. The gene for IAP1, the most abundant protein covalently modified by IAA (iap1, GenBank accession no. <ext-link ext-link-type="gen" xlink:href="AF293023">AF293023</ext-link>) was isolated and cloned from bush bean (Phaseolus vulgaris) seeds. The 957-bp sequence encodes a 35-kDa polypeptide. IAA-modified proteins represent a distinct class of conjugated phytohormones and appear in bean to be the major form of auxin in seeds. IAA proteins also are found at other stages of development in bean plants. Our immunological and analytical data suggest that auxin modification of a small class of proteins may be a feature common to many plants.