In breast cancer cell lines, the maturation of pro-cathepsin D into enzymatically active cathepsin D is altered, leading to its increased secretion. In order to specifically assay pro-cathepsin D (52 kD form) in breast cancer cytosol, we monitored a solid phase sandwich radioimmunoassay using D9H8 and D7E3 monoclonal antibodies raised against human pro-cathepsin D from MCF7 cells. Pro-cathepsin D was assayed in 108 primary breast cancer cytosols in which total cathepsin D was previously found to be correlated with metastasis. Pro-cathepsin D concentrations were found to be correlated with total cathepsin D and with lymph node invasion, and was slightly higher in premenopausal patients. By contrast, Cox multiparametric analysis showed that pro-cathepsin D status had no prognostic value for survival, or metastasis free survival contrary to total cathepsin D status. This first study shows the technical validity of the pro-cathepsin D assay but indicates that it has less value as a prognostic marker than total cathepsin D. This study also shows that the proportion of pro-cathepsin D recovered in vivo (1-6%) is much less than that produced in cell lines and suggests that the secreted pro-enzyme might be activated in the tumour extracellularly or following its reinternalisation.