An IgA1-specific lectin, Jacalin, was isolated from dried seeds of the jackfruit, Artocarpus integrifolia, by affinity binding to IgA1-Sepharose and elution with D-galactose. Jacalin is a glycoprotein with two non-covalently bound subunits (15 and 18 K). Interactions between Jacalin and human Igs were studied by precipitation in gel and in solution, and by agglutination of IgA1-coated latex by Jacalin. Jacalin precipitated only with IgA1-containing samples, including monomers, polymers, monoclonal, polyclonal and secretory IgA1, but not IgA2 of both A2m(1) and A2m(2) allotypes, nor with IgG1, 2, 3 and 4, IgM, IgD, and IgE; after neuraminidase treatment, only IgA1 and IgD were precipitated. Jacalin had a relatively broad pH range of activity in both precipitation and agglutination of IgA1-latex. Bivalent metal cations (Ca, Mg, Mn, Cu, Zn, Co, Cd), EDTA, Triton X-100, Tween-20, Na deoxycholate and ionic strength did not influence these reactions. Na dodecylsulphate, guanidine and urea inhibited the reactions whereas NP-40 rather enhanced them. Among 39 types of sugar tested, 10 displayed inhibitory activity, decreasing in the following order: p-nitrophenyl-alpha-D-galactopyranoside, 1-O-methyl-alpha-D-galactopyranoside, D-melibiose, p-nitrophenyl-beta-D-galactopyranoside, GalNAc, stachyose, 1-O-methyl-alpha-D-mannopyranoside, D-galactose, D-galactosamine and 1-O-methyl-alpha-D-glucopyranoside. IgA1, treated with neuraminidase or not, but not the other human Igs, was also an excellent inhibitor of agglutination, being more powerful than the best sugars studied. Only neuraminidase-treated IgD was also inhibitory, but less so than IgA1. Jacalin preferentially bound to alpha-linked non-reducing D-galactose. The configuration of OH-groups at C-2, C-4 and C-6 of D-galactose was important for the reaction. Jacalin recognizes terminal Gal beta 1-3GalNac-, as in the IgA1-hinge, and/or GalNAc-, but not Gal beta 1-4GlcNAc-, nor Gal beta 1-6GlcNAc-, nor their sialylayted extensions. Latex agglutination and its inhibition assay are particularly well suited for the study of these lectin-glycoprotein interactions.