Ly-49A+ murine natural killer (NK) cells cannot lyse target cells that express H-2Dd. We demonstrate a functional requirement for carbohydrate recognition by Ly-49A. Treatment of H-2Dd+ target cells with tunicamycin prevents their binding to Ly-49A+ cells and renders them susceptible to lysis by Ly-49A+ NK cells. Fucoidan, a sulfated polysaccharide, binds to Ly-49A in a calcium-dependent manner, and this binding is inhibited by monosaccharides, particularly sulfated hexoses. The inactivation of Ly-49A+ NK cells by H-2Dd+ target cells is reversed in the presence of glucose 6-SO4. These results indicate that Ly-49A has a functional carbohydrate recognition domain and that target expression of carbohydrates alters their susceptibility to natural killing.