Nicotiana tabacum was transformed with cDNA encoding the human cytosolic copper/zinc superoxide dismutase (human SOD) under the control of the cauliflower mosaic virus (CaMV) 35S promoter. The cDNA sequence was present in a range of copies, from single to several copies, in the primary transformant plants and a transcript of the expected length was produced. The expression of human SOD protein in the transgenic plants was shown by in situ gel staining assay. Ten plants produced human SOD up to 40 ng protein per mg of fresh leaf tissue.