The present invention relates to the identification of the active domain of Herpoxin, a DNA virus-inhibiting-protein which was isolated from cobra venom in U.S. Pat. No. 5,648,339 and has a molecular weight of 13.5 kDa We have isolated a fragment of Herpoxin which contains the active domain and which we have named Herp. Herp mimics the activity of Herpoxin in inhibiting the replication of DNA viruses. A synthetic version of the active fragment was produced having the amino acid sequence Asn-Leu-Tyr-Gln-Phe-Lys-Asn-Met-Ile-Gln. The synthetic version of Herp consisting of ten amino acids inhibits the replication of DNA viruses such as herpes viruses types 1 and 2, cytomegalovirus and varicella zoster virus as well as Tubercle bacilli.