1. Concanavalin A formed precipitates with polyelectrolytes such as fucoidan, RNA, heparin and bacterial lipopolysaccharides. 2. Precipitate formation also occurred between ficoll and concanavalin A. 3. Precipitate formation between concanavalin A and dextran or soluble starch was induced by the incorporation of phosphate groups into the unreactive glucans. 4. Introduction of polar groups, such as acetate, formate and phosphate, into glycogen resulted in enhanced precipitation with concanavalin A, whereas the opposite effect was noted on incorporation of hydrophobic (methyl) centres. 5. Neutral sugars and salt partially inhibited complex-formation between polyelectrolytes and concanavalin A. 6. Concanavalin A-glycogen complexes could be dissociated with 5% (w/v) trichloroacetic acid or 44% phenol-water. 7. Concanavalin A lost its glycogen-complexing ability after phenol treatment. 8. Evidence is presented for the existence of common binding sites on concanavalin A for both neutral polysaccharides and polyelectrolytes. 9. Hydrogen bonding appeared to play a major role in neutral polysaccharide-concanavalin A precipitate formation, whereas both hydrogen bonding and electrostatic forces were implicated in polyelectrolyte-concanavalin A complex-formation.