Bovine seminal ribonuclease (BS-RNase) and bovine pancreatic ribonuclease (RNase A) have potent antitumour activity. In this study, the antitumour effect of selected low-toxic antiangiogenins of natural origin and bovine ribonucleases conjugated with polyethylene glycol (PEG) was tested. The antiangiogenins were selected according their toxicity. The antitumour activity of PEGylated BS-RNase and RNase A was enhanced when injected together with antiangiogenins into the athymic nude mice bearing human pancreatic tumours and human melanoma C-32 tumours.