A rapid assay was developed to screen for herpes simplex virus (HSV) isolates that are resistant to acyclovir and other antiviral agents. The assay is a modified plaque reduction assay (PRA) in which the number of plaques seen in the absence of acyclovir was compared with that seen in the presence of a single cutoff concentration of acyclovir (2 microg/mL). This assay utilizes a cell line that expresses beta-galactosidase only after infection with HSV. Since histochemically stained plaques are easily visualized, small plaques can be easily enumerated. This allows the assay to be performed on dilutions of untitered specimens in the small wells of a 24-well plate and allows the results to be read only 2 days after inoculation of the virus. The assay performed well compared with a standard PRA and should be a valuable tool in identifying drug-resistant HSV in a timely manner.