An insect antiviral compound was purified from Heliothis virescens larval hemolymph by gel-filtration high pressure liquid chromatography (HPLC) and C-18 reverse-phase HPLC and its structure was determined by mass spectrometry. The antiviral compound is an N-myristoylated-peptide containing six amino acids with calculated molecular weight of 916 Da. The N-terminus contains the fatty acid myristoyl, and the C-terminus contains histidine with two methyl groups giving the histidine a permanent positive charge. The remainder of the compound is essentially non-polar. The structure of the compound corresponds with the 'myristate plus basic' motif expressed by certain viral proteins in their binding to the cytoplasmic side of the plasma membrane to initiate viral assembly and budding from a host cell. The insect antiviral compound may inhibit viral assembly and/or budding of viruses from host cells that could include the human immunodeficiency virus-1 (HIV-1) and herpes simplex virus-1 that use this motif for exit from a host cell. Using the formazan assay, the myristoylated-peptide was effective against HIV-1, with a nine times increase in the viability and protection in vitro of treated CEM-SS cells when compared with infected but untreated control cells. (C) 2004 Elsevier Inc. All rights reserved.