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Auxin activity was detected in honeydew obtained from the aphid Tuberolachnus salignus (Gmelin) feeding on willow (Salix viminalis). Active uptake of (14)C-indolyl-3-acetic acid (IAA) into the sieve tubes was demonstrated by irrigating the cambial surface of willow bark with (14)C-IAA solution and assaying aphid stylet exudate. When, however, (14)C-IAA was applied to the peridermal tissues of the bark or to a mature leaf most of the radioactivity (collected in honeydew or stylet exudate) co-chromatographed with indolyl-3-acetyl-aspartic acid (IAAsp). The presence of IAAsp in honeydew was not affected by extraction procedure or by aphid metabolism. Honeydew obtained from willow treated with (14)C-tryptophan contained only (14)C-tryptophan. When (14)C-IAA was applied in agar to the cut end of willow segments the radioactivity was found to move in a basipetally polar manner. The direction of movement of radioactivity in the sieve tubes, however, was found to be influenced by the proximity of the roots. Nevertheless, there was evidence that endogenous auxin in the sieve tubes does move in a predominantly basipetal direction.