Human recombinant superoxide dismutase (SOD), purified to homogeneity, is resolved by both conventional isoelectric focusing and immobilized pH gradients into three bands, with isoelectric points (pls) in the pH range 4.8 to 5.1, the p14.80 form representing the minor component. Due to the fact that this enzyme is expressed in E. coli, N-terminal acetylation or glycosylation should be ruled out. When purified by small-scale preparative isoelectric focusing in immobilized pH gradient gels, it was found that, upon subsequent analysis, the p l 5.07 form would band in the same position, but the intermediate p l 4.92 band would split into the upper (pl5.07) and the lower (pl4.80) species, in nearly the same amounts, whereas the lowest p l component would always generate both the intermediate and upper forms. Enzymatic essays pointed out that these three isoforms had nearly the same specific activity, slightly higher than that of the starting material. Metal analysis indicated that all three forms contained the same metal/protein ratio, approaching the value Cu,Zn,-SOD, as reported in the literature. Circular dichroism spectra of the p l 4.80 and 5.07 forms showed the same profile in the 190-240 nm range, but marked differences in the 250-350 nm region. Treatment with EDTA produces 1-2 additional, slightly higher p l isoforms, whereas treatment with KCN generates a number of higher p l components, reaching p l values as high as pH 7, with nearly complete disappearance of the three major SOD isoforms. It is concluded that these three isoforms could represent interconvertible species, the highest p l component representing the most stable conformer.