Imiquimod (also known as R-837 and S-26308) is an imidazoquinoline immune response modifier and is available in the US and several other countries for the treatment of external genital warts. Imiquimod has no direct antiviral activity but demonstrates efficacy in several animal models of virus infection. The drug is recognized by antigen presenting cells including monocytes, macrophages, B-cells and dendritic cells and induces these cells to produce cytokines including interferon-alpha (IFN-alpha) and others. Imiquimod's ability to inhibit primary lesion development in the guinea pig model of Herpes simplex virus (HSV) intravaginal infection was studied. Imiquimod given intravaginally reduced primary lesions, reduced virus shedding and reduced virus content of spinal cords from HSV infected guinea pigs. A single drug application of 0.5 mg/kg reduced lesion frequency when given between 24 h before inoculation to 16 h after inoculation. A single drug application of 5 mg/kg reduced lesion frequency and severity when administered between 72 h before inoculation to 24 h after inoculation. The antiviral effect resulting from interferon induction in the animal lasts much longer than the drug itself, thus imiquimod is different than drugs having direct antiviral activity. Twice daily drug application for 4 days was effective when initiated up to 72 h after inoculation, however, once lesions began to appear, imiquimod treatment was not able to stop lesion development. Imiquimod treatment inhibited lesion development and/or virus shedding in guinea pigs inoculated with HSV-1, HSV-2 or virus isolates resistant to acyclovir. Imiquimod is currently in clinical trials for treating human HSV infections.