This paper reviews the extensive and previously unpublished work on the interactions between agarose and 1,4-linked β- d-glycans carried out at Unilever Research, Colworth Laboratory, UK. The effect of the following variables is discussed: (i) galactose content of galactomannans; (ii) substitution patterns in the agarose molecule; (iii) structural variations in the 1,4-β- d-glycan main chain; and (iv) molecular size of the 1,4-β- d-glycans. Double helices of agarose, a non-substituted regular polysaccharide comprising 1,3-linked β- d-galactose and 1,4-linked 3,6-anhydro-α- l-galactose, bind in an ordered cooperative fashion to an extended ribbon ordered conformation of sequences of 1,4-linked β- d-mannopyranose residues in plant galactomannans to give mixed gelling systems. This interaction survives, in a modified form, substitution along the agarose molecule by O-methyl ether and O-sulphate esters at O6 of the d-galactose and O2 of the 3,6-anhydro- l-galactose, and 4,6-linked pyruvic acid ketal groups on the d-galactose. The higher the level of substitution on the agarose, the weaker the interaction with galactomannan. In general, the higher the level of galactose substitution in the galactomannan the lower the extent of interaction with agarose. Evidence is presented, however, which indicates that the fine structural distribution of galactose along the galactomannan molecule is also an important determinant for the co-gelling interaction. Substituted 1,4-linked β- d-glucomannans, β- d-glucans and β- d-xylans which can form closely similar extended ribbon order conformations to the galactomannans also participate in co-gelling interactions with agarose. These β- d-glycans are similar in structure to important skeletal polysaccharides such as hemicelluloses and cellulose. This suggests that the binding between agars and β- d-glycans might mimic biological cohesion between skeleton and gel phases in natural red seaweed cell walls. The sensitivity of the interactions studied to fine details of agar and β- d-glycan structure is what might be expected on biological grounds, since the wide and subtle variations of natural polysaccharide structure are presumed to represent a mechanism for control of their intermolecular interactions.