Dextran-binding immunoglobulin (DIg) was detected in circulating immune complexes in young healthy blood donors. High molecular weight dextran or non-dialysable polysaccharides of edible sugar added to serum markedly increased DIg-containing immune complexes. DIg from plasma was purified by an improved affinity chromatography using Sephadex G-200 to increase yield. Recognition by DIg of alpha (1-->6) glucans in polystyrene-coated dextran was inhibited totally and its less avid recognition of beta (1-->3) glucans in yeast and barley partially by 50 mM 1-O-methyl-alpha-D-glucoside. All fractions of non-dialysable edible sugar separated by electrophoresis were recognized by DIg. While purified DIg contained nearly equal amounts of IgM and IgG, DIg in immune complexes isolated after addition of dextran to serum was 72% IgM. Recognition of bovine thyroglobulin, tumour laminin, and peanut-agglutinin-binding bovine heart glycoproteins, all containing terminal alpha (1-->3)-linked galactose epitopes, by DIg was inhibitable by 1-O-methyl-alpha-D-glucoside. Enzymatic removal or modification of the respective terminal alpha (1-->3)-linked galactose groups in these glycoproteins abolished their recognition by the antibody. Results suggest that DIg recognizes alpha (1-->6) glucans, alpha (1-->3) - linked galactose epitopes and less avidly beta (1-->3) glucans. Results also point to a new immunomodulatory effect of non-dialysable sugar and dietary beta-glucans.