The role so far ascribed to intracellular CuZn superoxide dismutase is that of an intracellular scavenger of oxygen radicals. However, other functions of cytosolic CuZn superoxide dismutase have been hypothesized. For example, CuZn superoxide dismutase incubated with rat hepatocyte cells in culture inhibits 3-hydroxy-3methylglutaryl CoA reductase, thereby reducing cholesterol synthesis. We recently demonstrated the presence of surface membrane receptors for CuZn superoxide dismutase, suggesting possible autocrine or paracrine activities. The aim of the present study was to investigate whether cytosolic CuZn superoxide dismutase can be secreted by human hepatocarcinoma and fibroblast cells lines. Proteins in human hepatocellular carcinoma (Hep G2) cells and human fibroblasts were biosynthetically labelled with [35S]-cysteine; then cell lysates and media were immunoprecipitated with rabbit polyclonal anti-human CuZn superoxide dismutase antibodies and separated by 12% polyacrylamide gel electrophoresis. Both Hep G2 cells and human fibroblasts produce and secrete CuZn superoxide dismutase which was detectable in cells and medium as a single protein band with the same electrophoretic mobility as human erythrocyte CuZn superoxide dismutase. These data suggest that CuZn superoxide dismutase, an enzyme thus far considered to be located exclusively intracellularly is secreted by at least two cell lines. This is consistent with autocrine or paracrine roles for CuZn superoxide dismutase.