Nahmias, André J. (Boston University School of Medicine, Boston, Mass.), and Sidney Kibrick. Inhibitory effect of heparin on herpes simplex virus. J. Bacteriol. 87:1060–1066. 1964.—A substance inhibitory to herpes simplex virus was observed during experiments with leukocyte cultures. The component in the cultures responsible for this inhibition was identified as heparin. The minimal inhibitory concentration required to inhibit 30 to 300 tcd50 of the virus in human amnion tissue culture was found to be 1 to 2 units per ml (10 to 20 μg/ml). This effect was confirmed with other strains of herpes simplex virus, other tissue-culture systems, and other media. The inhibitory activity of the heparin was found to be related to the sulfate groupings on the molecule. The effect of heparin appears to be on the virus, rather than on the cell. The virus is not inactivated, however, and the heparin-virus “complex” is readily dissociable on dilution. Heparin was shown to affect viral infection in its earliest phase, probably at the primary electrostatic attachment of virus to cell. The import of these and related observations on common virological laboratory procedures and the possible biological significance of our findings are discussed.