Mitogenic properties have been demonstrated in vitro for the lysosomal acidic protease cathepsin-D (cath-D). We investigated possible relationships between cath-D cytosol cell content and tumor proliferative activity in a series of 129 operable breast cancer patients. For total cytosol cath-D evaluation, a solid phase two-site immunoradiometric assay was utilized on tumor cell cytosol obtained for hormone receptor assay (DCC method). The percentage of S-phase cells was analyzed by 3H-thymidine autoradiographic assay. Median 3H-thymidine Labeling Index (3H-Tdr-LI) of the series was 2.7%; median cath-D content resulted 57 pmol/mg of protein cytosol and was significantly higher in node-positive with respect to the node-negative subgroup (p < 0.03). When classified in low, intermediate or high tumor cath-D content and slow or fast proliferative activity (cut-off: median values of the series), no significant agreement was found between the two variables. Statistical analysis, however, showed that a significant inverse correlation existed in node positive tumors between cath-D and 3H-Tdr-LI values which was even more evident in N-positive high estrogen receptor-positive (ER+) cases (coefficient of correlation = 0.6828; p = 0.0001). Cytosol cath-D content cannot be generally proposed as a direct marker of proliferative activity for operable breast cancer.