Comparisons of the anti-herpes virus activity of idoxuridine, cytarabine, vidarabine, trifluorothymidine and ara-DAP were made in vitro and in vivo . Although the activity of these compounds was substantiated in vivo , the test methods were insufficiently sensitive for adequate comparisons of relative activities to be made. It was found that such comparisons are best made in vitro by the plaque reduction technique, by which cytarabine was found to be the most active compound on a molar basis available at the present time.