Abstract:Change in the concentration of endogenous IAA was measured with enzyme-linked immtmosorbent assay (ELISA), and the distribution of IAA-binding protein in the protoplasts in the cambial region was tested with an immuno-fluorescence technique, being improved by the authors, during the cambial active period in Broussonetia papyrifera (L.) Vent. The results showed that the concentration of the endogenous IAA increased rapidly as the immature xylem and phloem were luxuriantly formed from the cambium, and decreased and maintained at a certain lower level when the immature vascular cells differentiated toward maturation. The IAA- binding protein was mainly distributed in the plasmalemma, protoplasm, nuclear envelope and nuclear substance.