Changes in the expression and processing of cathepsin D (CD) have been shown to be associated with cancer invasion and metastasis. However, the value of CD as a prognostic marker remains controversial. Most studies have used immunological methods to measure the mature form of CD, although it is the precursor (pro-CD) that appears to be abnormally secreted by breast cancer cells. A sandwich-type ELISA has been developed that is specific for pro-CD. The assay employs a monoclonal antibody to mature CD as the capture reagent and a rabbit polyclonal to the pro fragment as the detector. The assay is specific for pro-CD and capable of quantitating this antigen in biological samples. Pro-CD levels were measured in plasma samples from 76 breast cancer patients and compared with 36 samples from normal control individuals. The plasmas of breast cancer patients showed elevated levels of pro-CD; 12% were more than two standard deviations above the mean for the normal samples. Immunoblots of the plasma samples using a CD monoclonal antibody revealed a band at the appropriate size for pro-CD that corresponded in intensity with the ELISA results. Affinity adsorption of breast cancer plasmas with pepstatin agarose followed by immunoblot analysis revealed a single protein band that correlated with pro-CD. Only trace amounts were detected in the normal control plasmas. These results demonstrate that cathepsin D is present in the plasma of breast cancer patients primarily in its precursor form and that it may be a useful prognostic indicator.