In some families with amyotrophic lateral sclerosis (ALS), the disease is linked to mutations in the gene encoding CuZn-superoxide dismutase. The mutant CuZn-superoxide dismutases appear to cause motor neuron degeneration by a toxic property, suggested to be linked to an altered reactivity of the active-site Cu ions. Asp90Ala mutant CuZn-superoxide dismutase was isolated from six patients with ALS, allowing properties of the mutant enzyme synthesized and conditioned in patients with ALS to be examined. The molecular mass of the Asp90Ala mutant CuZn-superoxide dismutase was 45 Da lower than that of the wild-type enzyme, as expected from the amino acid exchange. The mobility after sodium dodecyl sulfate-polyacrylamide gel electrophoresis was markedly increased, however, suggesting altered properties of the polypeptide. The mutant CuZn-superoxide dismutase showed a minimal reduction in stability but did not differ significantly from the wild-type enzyme in enzymic activity, in content and affinity for active-site Cu ions and in the propensity to catalyze formation of hydroxyl radicals. Our findings suggest that the deleterious effect of mutant CuZn-superoxide dismutases on motor neurons in ALS is not related to altered reactivity of active-site Cu ions, resulting in increased oxidant stress. Attention should therefore also be directed at other mechanisms and properties of the mutant polypeptides and their degradation products.