Recombinant cellulose-binding domain (CBD) was found to modulate the elongation of different plant cells in vitro. In peach pollen tubes, maximum elongation was observed at 50 ug/ml CBD. Pollen tube-staining with calcofluor indicated loss of crystallinity in the tip zone of CBD-treated pollen tubes. At high concentrations (100-500 ug/ml), CBD dramatically inhibited root elongation in a dose-responsive manner. However, between 1 × 10-2-100 ug/ml CBD had the opposite effect on root hair elongation as compared to its effect on root elongation. CBD competes with xyloglucan (XG) for binding to cellulose. When CBD was added first to the cellulose, increasing concentrations of CBD resulted in increasing amounts of unbound XG. However, when XG was added first, increasing concentrations of CBD did not affect the level of unbound XG. The level of unbound CBD was higher when XS was added first to the cellulose.