Due to the emergence of drug-resistant mutants during the treatment of viral infections, an easy and rapid laboratory technique to assay the antiviral sensitivity of virus isolates could be extremely useful. In this brief report we describe the use of enzyme-linked immunoassay (EIA) to detect the susceptibility of herpes simplex virus (HSV) type 1 to three well known antiviral agents: Cytosine-arabinoside (Ara-C), 5'-iododeoxyuridine (IUDR), and acyclovir (ACG). The results indicate that EIA can easily give an accurate indication of antiviral susceptibility 24 hr after the isolation of the virus and provides important information from the therapeutical point of view.