An anatomical feature of the platypus (Ornithorhynchus anatinus) that is seen in only one other mammal, the echidna, is that the male has a crural glandular system that produces venom that is used for defence and territorial–breeding functions; whether the echidna is similarly venomous is not yet established. Platypus venom contains many novel proteins and peptides that are different from those in reptilian venom. It also causes pain and symptoms that are not experienced by any other kind of envenomation. Five types of proteins and peptides have been isolated and identified from platypus venom, namely: defensin-like peptides (DLPs); Ornithorhynchus venom C-type natriuretic peptides (OvCNPs); Ornithorhynchus nerve growth factor; hyaluronidase; and l-to-d-peptide isomerase. The structures of DLPs and OvCNPs have already been studied and they are very similar to β-defensin-12 and mammalian C-type natriuretic peptides, respectively. A special mammalian l-to-d-peptide isomerase that is responsible for interconverting the l- and d-peptide isomers is also found in platypus venom. Isomerase activity has recently been discovered in platypus tissues other than the venom gland. It is possible that similar kinds of enzymes might exist in other mammals and play important, as yet unknown, biological roles. Considering the fact that some animal venoms have already been widely used in pharmaceutical applications, research into platypus venom may lead to the discovery of new molecules and potent drugs that are useful biomedical tools.