A simple procedure for the affinity purification of procathepsin D from tissue culture medium conditioned by breast-cancer cells is described. This procedure yielded 2 μg of procathepsin D/100 ml medium. The procathepsin D was approximately 95% pure as judged by silver staining of polyactylamide gels, the major contaminant being mature cathepsin D. The ability of procathepsin D to stimulate the proliferation of oestrogen-responsive MCF-7 breast cancer cells was determined. The purified procathepsin D had no mitogenic effect alone or in combination with oestradiol or other growth factors. These data suggest that procathepsin D does not act as an oestrogen-regulated autocrine growth factor for malignant breast epithelial calls.