Previous studies on murine T cell IgD-R have shown that these receptors recognize N-glycans of murine IgD, and not of other Ig isotypes. We have now studied the specificity of IgD-R on human T cells. Human IgD digested with proteinase K to fragments of X 5 kDa inhibit the ability of T cells to form rosettes with IgD-coated ox erythrocytes. The same amount of digested IgG does not. We tested all the human Ig isotypes: IgG1,-2,-3,-4, IgA2, IgE and IgM fail to inhibit significantly at 20 ? g/assay. However, IgA1 is as effective as IgD itself, showing approximately 60 % and 80 % inhibition at 5 ? g and 10 ? g/assay. Human IgA1 and IgD both contain Gal-1 1 3-GalNac-rich O-linked glycans, and on this basis are both bound to ricin and jacalin. The O-linked glycans may therefore also represent the common moiety binding to IgD-R. Disaccharides Gal-1 1 3-GalNac, and Gal-1 1 4-Glc at 10 ? g/assay blocked IgD rosetting while Gal-1 1 6-Glc did not. We conclude that the human IgD-R is a lectin, differing from the murine IgD-R in that it has both IgA1 and IgD as ligands.