The quantitatively major nonasaccharide (XG9) derived from xyloglucan by digestion with cellulase exhibits anti-auxin activity in the pea stem segment straight-growth bioassay; the most effective concentration of XG9 is c. 10-9 M. Previous work had shown that XG9 owes its biological activity to the presence of a terminal α-L-fucopyranose residue. In order to investigate to what extent the remainder of the XG9 molecule is essential for activity, several fucose-containing compounds were tested for their ability to mimic the anti-auxin effect of XG9. A fucose-containing pentasaccharide of xyloglucan (XG5; probable structure Fuc→Gal→Xyl→Glc→Glc) was, at 10-8 M, about as effective an anti-auxin as 10-9 M XG9; unlike XG9, XG5 did not diminish in effectiveness at 10-7 M. The human milk trisaccharide, 2'-fucosyl-lactose [L-fucopyranosyl-α-(1→2)-D-galactopyranosyl-β-(1→4)-D-glucose], whose Fuc→Gal unit is identical with that of XG9, inhibited auxin-induced elongation over a wide range of concentrations centred on about 10-8 M. 2'-Fucosyl-lactose at 10-8 M was about as effective an anti-auxin as 10-9 M XG9. Free L-fucose and methyl-α-L-fucopyranoside were unable to inhibit auxin-induced growth at any concentration tested (10-10 M to 10-6 M) and neither compound interfered with the inhibition caused by 10-9 M XG9 when co-incubated at concentrations up to 10-4 M. The results confirm the essential rôle of an α-linked terminal fucose residue in the anti-auxin activity of XG9 and show that the sub-terminal galactose residue may also be required. Possible reasons why high concentrations of XG9 fail to antagonize auxin-induced growth while high concentrations of XG5 and 2'-fucosyl-lactose continue to do so are discussed.