. The gene for human superoxide dismutase (hSOD), a cytoplasmic enzyme, was cloned into a high expression, secretion vector, pIN−III−OmpA. Upon induction of the gene expression, a new protein was produced at a level of approximately 10% of total cellular protein, which migrated at the identical position in SDS−polyacrylamide gel electrophoresis as hSOD produced in the E. coli cytoplasm. The product was recovered as the major component in the periplasmic fraction. The specific enzymatic activity of the secreted SOD was approximately half that of the cytoplasmic SOD expression with a non−secretion vector. These results indicate that a substantial fraction of the secreted SOD was able to be properly folded into the enzymatically active conformation. This is the first report to demonstrate that a foreign cytoplasmic protein can be secreted across the inner membrane into the periplasmic space yielding an active enzyme. .