Oligosaccharides with regulatory effects on living plant tissue have been obtained by parital hydrolysis of xyloglucan, cellulose and pectic polysaccharides. Attention is focused here on xyloglucan-derived oligosaccharides (XGOs), which exert the following two distinct effects on cell growth in pea-stem segments. (i) At approx. 1 nM, the L-fucosylated XGOs, such as XXFG, XFFG and FG (for structure of XXFG, see Fig. 1), antagonize 2,4-dichlorophenoxyacetic acid (2,4-D)-stimulated growth. At approx. 100 nM, XXFG loses this growth-inhibitory effect, probably because it gains a growth-promoting effect [see (ii)]; in contrast, FG retains its growth-inhibitory effect. The growth-inhibitory effect is tentatively attributed to membrane-binding of the active XGOs. (ii) At approx. 1 microM, at least four different cellotetraose-based XGOs (XXXG, XXLG, XXFG and XLLG) mimic auxin in that they induce growth. This effect is thus not L-fucose-dependent and is not exhibited by the cellobiose-based pentasaccharide, FG. Effect (ii) is attributed to the ability of cellotetraose-based XGOs to act as acceptor substrates for xyloglucan endotransglycosylase. [formula: see text] The biosynthesis and biodegradation of relevant XGOs has been investigated. By use of labelling with L-[3H]arabinose and L-[3H]fucose in vivo, XXFG and O-acetyl derivatives thereof were shown to accumulate extracellularly, in spinach cell cultures, to approx. 0.1 microM. The kinetics of labelling of XXFG showed it to be formed by degradation of pre-formed polysaccharide rather than by de novo synthesis of the oligosaccharide. XXFG was remarkably stable in vivo, undergoing little hydrolysis in contact with the surfaces of cultured cells; the major metabolic fate of exogenous [3H]XXFG was sequestration into apoplastic polysaccharide by endotransglycosylation.