The relationship between proteinase-like peptidase activities and oestrogen receptor levels and status in breast cancer tissue homogenates from 61 patients with breast cancer has been evaluated. With Spearman's rank-order correlation analysis, significant positive correlations were observed between receptor levels and the activities of cathepsin-(B + L)-like, cathepsin-H-like, trypsin-like, plasminogen-activator-like and elastase-like peptidases. In addition, the activities of all but the latter enzyme were significantly higher in patients with receptor-rich tumours than in receptor-poor tumours, and this may have implications for future treatment regimens for patients with oestrogen-receptor-rich tumours. The findings reported are consistent with the suggestion that in breast cancer there may be an association between steroid receptors and proteolytic enzymes such that the release of these enzymes may be under hormonal control.