Coupling of alginate with 1-amino-1-deoxygalactose in the presence of 1-ethyl-3-[3-(dimethylamino)propyllcarbodiimide results in a substituted polymer containing galactose side linked via an amide bond. To clarify the degree and pattern of substitution, a H-1 NMR study on the anomeric region of modified alginate, polymannuronate, alginate enriched in guluronic acid (G-enriched alginate), and polyalternating MG, was carried out (G, alpha-L-guluronic acid; M, beta-D-mannuronic acid). From the resonance of the proton at position 1 of galactosylamine, it was possible to determine the amount of galactose linked to mannuronic and to guluronic residues, respectively. Furthermore, H-1 NMR spectroscopy revealed a higher reactivity of guluronic residues for low degrees of conversion. Modified alginates with 7% and 19% of substitution are both able to form stable beads in the presence of calcium ions. The effect of galactose substitution on the dimensions, swelling,and stability of the beads has been studied and the cytotoxicity of the modified polymer evaluated in preliminary biological tests.