Abstract: The cellulase A(buffer-soluble)and B(salt-soluble)activities in abseission zone of fruit explants of "Anliu-cheng" orange(Citrus sinensis Osbeck)rose markedly during prenalure drop just after petal fall(first fruit drop),cellulase A activity in the abscission layers of fruit explant and attached fruit also increased significantly in the course of the shedding at the disc(second fruit drop),but there was little change in rellulase B during that time.2,4-D could inhibit not only the abscission of fruit explants collected in the two fruit drops just mentioned above,but also first fruit drop in the field by low concentration spraying,due to the marked depression of the exo-and endo cellular cellulase,cellulase A activity and ethylene production.But it had no significant effect on cellulase B activity in this experiment.The use of gibberellic acid in decreasing fruit shedding during the second drop was more effective than that of 2,4-D.The relationships among fruit abscission,cellulaseactivity and ethylene production and the regulating mechanism of 2,4-D on fruit drop were discussed.