The interaction of lactate dehydrogenase with high-molecular-weight derivatives of Cibacron Blue was followed by measuring the reaction kinetics and by means of affinity electrophoresis. The dye was covalently bound to dextrans with relative molecular mass ranging from 5 000 to 2 000 000 and to hydroxyethyl-starches with relative molecular mass from 130 000 to 2 000 000. In the case of dextran derivatives of Cibacron Blue, various degrees of substitution by the dye were tested. Measurement of kinetics showed that the Cibacron Blue derivatives competed with NADH for the binding site on the enzyme and that when the dye was bound to a polysaccharide, the affinity of lactate dehydrogenase to the dye decreased. Both methods used confirmed that the strength of interaction did not depend on the relative molecular mass of the polysaccharide carrier or on the degree of substitution. The interaction of lactate dehydrogenase with hydroxyethyl-starch derivatives of Cibacron Blue was weaker than with dextran derivatives.