Throughout most of his research career, Peter has focused on protein-degrading enzymes (proteases) and their role in cancer in humans and in the fibrinolytic system that serves the breakdown of blood clots. In particular, Peter studied uPA (urokinase-type protease plasminogen activator) and PAI-1 (its natural inhibitor) and has contributed significantly to understanding their importance to the cancer cell's ability to form metastases. The organization, American Society for Clinical Oncology, now recommends measurement of these components for breast cancer prognosis. The primary aim of the work carried out by Peter's lab has been to understand the details of the molecular mechanisms underlying these processes. Therefore, Peter established a number of biophysical technologies, which have since been to the benefit of many research groups at the institute.

At the age of 62, Peter added an extra dimension to his research by establishing and spearheading a Danish-Chinese Centre for Proteases and Cancer, with support from the Danish National Research Foundation. Through this cooperative, which involved extensive exchanges of personnel, including Peter himself, he managed to provide valuable structural information about protease binding to their respective substrates and inhibitors. In the same period, Peter attracted two large grants from the National Research Council. This was an especially impressive late addition to an already very strong research effort. It is thus not surprising that Peter was a researcher at the institute which has been cited most frequently by other researchers. More than 14 000 scientific articles have referred to Peter's scientific publications.