Hydrolysis of extracellular matrix is a necessary step for malignant cells to invade, and metastasize. Three groups of proteinases, mainly serine, thiol and metalloproteinases, have been found to be secreted by cancer cells and responsible for the proteolytic cascade triggered during invasion. Previous studies from our group and others have shown that the thiol proteinase cathepsin B1 is a constant indicator of tumor invasion in carcinoma of the cervix, although others point to plasminogen activators and collagenases. So far, there are no systematic studies to correlate cathepsin B and plasminogen activator activity with advancing malignant disease and thus estimate its capability as a marker of progression. The purpose of this study was to determine the activity of cathepsin B like proteinase and plasminogen activators in invasive carcinoma of the breast at various clinical stages and with different estrogen receptor status. One hundred patients with carcinoma of the breast at different clinical stages were studied. Cathepsin B and plasminogen activators activity was assessed in tumor cytosols using different synthetic oligopeptides as substrates following the method of Smith. Estrogen receptor concentration was determined with monoclonal antibodies. A statistical analysis and correlation with different clinical stages was performed. Cathepsin B-like activity had a consistent and progressive elevation in direct correlation with clinical stage (stage I, 1.97 SE +/- 0.46; stage II, 6.67 SE +/- 1.12; stage III, 28.19 SE +/- 3.48; nmol/mg/30 min), while plasminogen activators, although constantly elevated, had no correlation with tumor progression. No relation could be found with estrogen receptor status. It is concluded that cathepsin B, but not plasminogen activator, is a good indicator of tumor progression in invasive carcinoma of the breast.