The carbohydrate-binding properties of the Datura stramonium seed lectin were studied by equilibrium dialysis, quantitative precipitation of natural and synthetic glycoproteins, and hapten inhibition of precipitation. The dimeric lectin (Mr = 86,000) possesses two carbohydrate-binding sites for N,N'N",N"'- tetraacetylchitotetritol /mol protein, with an apparent Ka = 8.7 X 10(3) M-1 at 4 degrees C. Whereas fetuin and orosomucoid reacted poorly with the Datura lectin, the asialo derivatives of these glycoproteins gave strong precipitation with the lectin. Carcinoembryonic antigen, type 14 pneumococcal capsular polysaccharide, and bovine serum albumin, highly substituted with N,N'- diacetylchitobiose units, also precipitated the lectin. Of the homologous series of chitin oligosaccharides tested, N,N',N"'- triacetylchitotriose was over 6-fold more potent than the disaccharide (N,N'- diacetylchitobiose ) which, in turn, was 90 times more reactive than N-acetyl-D-glucosamine. N-Acetyllactosamine [beta-D-Gal-(1----4)-D-GlcNAc] was also a potent inhibitor of Datura lectin being equivalent to N,N'- diacetylchitobiose . The requirement for an N-acetyl-D-glucosaminyl unit linked at the C-4 position was established. The biantennary pentasaccharide (penta-2,6) was a 500-fold more potent inhibitor than N-acetyllactosamine, suggesting that it might interact with both saccharide-binding sites of the Datura lectin simultaneously.