Cathepsin D, an aspartic lysosomal proteinase, is overexpressed by breast cancer cells and highly correlated with the occurrence of metastasis in patients. We used an adenovirus transformed rat cell line that does not secrete cathepsin D, to study the consequences of transfecting human cathepsin D cDNA. Overexpression of human cathepsin D in stable transfectant clones, results in higher initial growth rates in low serum conditions, overgrowth at high cell densities resulting in stellate aggregates, and greater anchorage-independent growth in soft agar. The metastatic activity (mostly in liver) of cathepsin D clones injected into athymic mice was significantly higher than that of control clones. These results show that overexpression of cathepsin D increases the transformed phenotype of malignant cells in vitro and their metastatic potency in vivo.