Seedlings of Arabidopsis thaliana were germinated and grown in medium containing beta-glucosyl Yariv reagent (beta GlcY), a synthetic phenyl glycoside that interacts specifically with arabinogalactan-proteins (AGPs), a class of plant cell surface proteoglycans. The effect of beta GlcY on the seedlings was to reduce the overall growth of both the root and the shoot. beta GlcY only accumulated in the root tissues and the reduced growth of the shoot appeared to be an indirect effect of impaired root growth. Reduced root growth was a consequence of a reduction in cell elongation during the postproliferation phase of elongation at the root apex and this was associated with extensive radial expansion of root epidermal cells. beta GlcY penetrated roots as far as the endodermis and it is suggested that the interaction of beta GlcY with AGPs in the load-bearing cell layers inhibited root elongation. When beta GlcY was added to carrot suspension-cultured cells that had been induced to elongate rather than proliferate, cell elongation was inhibited. The AGP-unreactive alpha-galactosyl Yariv reagent (alpha GalY) had no biological activity in either of these systems.