The Lewis X (Le(X)) determinant, a trisaccharide with the carbohydrate sequence Gal beta(1 -> 4)[Fuc alpha(1 -> 3)]GlcNAc beta, is believed to be responsible for Ca2+-mediated cell-cell interactions. In partly oriented phases composed of mixtures of penta(ethyleneglycol)monododecyl ether HO(CH2CH2O)(5)C12H25 and n-hexanol in the presence of Ca2+ ions, the variation of the residual dipolar couplings D-1(CH) of various CiHi vectors in Le(X) as a function of the concentration of the trisaccharide demonstrates the existence of very weak Le(X)-Ca2+-Le(X) complexes in solution. Synthetic 3-, 4-, and 6-deoxy-Le(X) variants were also shown to form complexes in the presence of calcium ions, despite the replacement of one of their hydroxyl groups by hydrogen atoms. This is the first direct observation in solution of a calcium-mediated interaction between Le(X) molecules.