The effects of the administration of normal human plasma to patients affected by mucopolysaccharidoses I and II (Hurler's and Hunter's syndromes) have been evaluated. The infusion was followed by a decreased urinary excretion of relatively large molecular weight glycosaminoglycans and by an increased excretion of their products of degradation. Among the latter, products of the degradation of dermatan sulfate and heparan sulfate could be demonstrated. The results indicate that normal human plasma may contain those "factors" that are involved in the normal degradation of dermatan sulfate and heparan sulfate, that are missing in the diseased states.