Superoxide dismutase in human erythrocytes was assayed by the inhibition of highly diluted erythrocyte lysates on 2-methyl-6-(p-methoxyphenyl)-3,7-dihydroimidazol[1,2-alpha]pyra zin-3-one- dependent luminescence induced by the hypoxanthine-xanthine oxidase system. Our chemiluminescence procedure gave 95-times higher sensitivity than the cytochrome c method. The concentration of superoxide dismutase in erythrocytes of Down's syndrome patients was approx. 1.8-times higher than that of normal humans.