Human cupro-zinc superoxide dismutase is polymorphic in northern Sweden. The genetic variant type has a lower mobility at electrophoresis in alkaline buffer. The enzyme was isolated from erythrocytes from one of the rare homozygotes and its properties compared to those of the common type. The isoelectric point of the variant was higher (4.85) than that of the common type (4.7). Small differences in amino acid composition were found but no definite amino acid substitutions could be pointed out. The molecular weights were equal as judged from electrophoreses in polyacrylamide gels in the presence of dodecylsulphate. The ultraviolet spectra were similar. Parameters related to the active site of the enzyme were very similar; i.e. specific activity and sensitivity to inhibition by cyanide and by H2O2. These parameters, especially the latter two, differ widely between species. Both enzymes were stable for weeks at neutral pH at 37 degrees C, whereas the common type was significantly more stable at pH 4 and pH 11 and also at incubation in neutral buffer at 70 degrees C. It appears that the active site of the variant is conserved whereas the stability of the enzyme is affected.