We have prepared a monoclonal antibody (MAb), 3D3, raised against purified human respiratory mucins. This antibody recognized mucins and proteolytically derived glycopeptides. The epitope recognized by the antibody was destroyed by alpha-L-fucosidase, indicating that it was present on the carbohydrate moieties. Structural specificity was determined by adsorption on a variety of synthetic, insolubilized oligosaccharides. Several lines of evidence indicate that the 3D3 MAb reacted strongly with the Lewis (Leb) antigen, but also recognized Le(a) and Le(y) determinants. This antibody might be useful to study mucin secretion.