Arabinogalactan-proteins (AGPs) are abundant plant proteoglycans that react with (beta-D-Glc)3 but not (beta-D-Man)3 Yariv reagent. We report here that treatment with (beta-D-Glc)3 Yariv reagent caused inhibition of root growth of Arabidopsis thaliana (L.) Heynh. seedlings. Moreover, the treated roots exhibited numerous bulging epidermal cells. Treatment with (beta-D-Man)3 Yariv reagent did not have any such effects. These results indicate a role for AGPs in root growth and control of epidermal cell expansion. Because treatment with (beta-D-Glc)3 Yariv reagent phenocopies the reb1 (root epidermal cell bulging) mutant of Arabidopsis, AGPs were extracted from the reb1-1 mutant and compared with those of the wild type. The reb1-1 roots contained an approximately 30% lower level of AGPs than the wild type. More importantly, while the profile of AGPs from wild-type roots showed two major peaks upon crossed electrophoresis, the profile of AGPs from reb1-1 roots exhibited only one of the major peaks. Therefore, the reb1 phenotype appears to be a result of defective or missing root AGPs. Taken together, this pharmacological and genetic evidence strongly indicates a function of AGPs in the control of root epidermal cell expansion.