Abstract.Oligosaccharides that exert biological effects on higher plants (other than as carbon or energy sources) have been termed‘oligosaccharins'. A limited number of specific oligo‐β‐glucans derived from fungal cell walls exhibit oligosaccharin activity, at very low doses, switching on the synthesis of phytoalexins. The structural requirements for this biological activity are stringent, and there is strong evidence that the oligosaccharins of fungal origin act through a receptor in the plant cell membrane. Chemically unrelated oligosaccharins can also be produced by partial digestion of pectins and xyloglucans from higher plant cell walls. In some cases, for example, the α‐l‐fucosylated oligoxyloglucans that antagonise the growth‐promoting effect of auxin, these plant‐derived oligosaccharins have relatively strict structural requirements for activity and act at ∼10−6mol m−3; these may act via specific receptors. In other cases, for example, the growth‐promoting action of other oligoxyloglucans, the stringency is somewhat lower and the activity is only seen at ∼10−3mol m−3, and these oligosaccharins are proposed to influence growth through their ability to modulate the activity of an enzyme, cellulase. A third class of plant‐derived oligosaccharins, the oligo‐α‐galacturonides, appear to have much less stringent structural requirements; a number of unrelated physiological responses are evoked by the same range of oligo‐α‐galacturonides at ∼10−3to 1 mol m−3. We suggest that these oligosaccharins may act via a mechanism not involving a specific receptor, perhaps by interacting with the plasma membrane to