(~) ELSEVIER Ann. Inst. Pasteur/ViroL Paris 1988 1988, 139, 185-195 EVALUATION OF HERPES SIMPLEX VIRUS SUSCEPTIBILITY TO ACYCLOVIR USING AN ENZYME-LINKED IMMUNOSORBENT ASSAY P.M. Andre, C.H. Narbonne, P.Y. Donnio, A. Ruffault and B. Fauconnier Laboratoire de Microbiologie, Facultd de Mddecine, A venue du Pr L. Bernard, 35043 Rennes Cedex (France) SUMMARY An/n situ ELISA was performed directly on the adherent cell monolayer in order to determine the susceptibility of herpes simplex virus isolates to acyclovir. Various fixation procedures and antisera conjugated to different enzymes were tested. The use of glutaraldehyde for fixation and beta- galactosidase as a labelling enzyme was shown to give the best results. As with other currently used assays, 50 % inhibitory doses were subject to an inoculum effect. The data obtained indicate that this assay is suitable for routine determination of herpes simplex virus susceptibility to antiviral drugs. KEY-WORDS: Herpes simplex virus, Acyclovir; ELISA, Evaluation. INTRODUCTION The increasing number of available drugs active against herpes simplex virus (HSV) and the emergence of resistant strains in vitro and in vivo (for review see [12]) require rapid and sensitive assays to evaluate the susceptibili- ty of viruses to antiviral compounds. The plaque reduction assay is the method most often used [7, 8, 10], but this assay involves much labour. Its measure- ment is based on a subjective reading and does not differentiate the plaque morphology or the importance and rate of viral replication within one pla- que. Colorimetric assays [5, 6] present the advantages of rapidity and the possibility of automating micromethods, but these assays give only low op- tical densities and operate negatively, detecting living cells left after infec- Submitted December 29, 1987, accepted March 19, 1988.