The incidence of genital herpes simplex infections has risen at a rapid rate over the past 20 years. Sexually active young adults ask whether condoms will effectively block the transmission of herpes simplex virus from one individual to another. Using a laboratory plunger model to stimulate human coitus, the authors have shown that high concentrations of HSV-2 did not cross the latex or natural membrane condoms that were tested. We have also shown that high concentrations of HSV-2 left in contact with a condom for 8 hr does not penetrate the barrier. This study gives good indirect evidence that condoms block the transmission of herpes simplex virus.