A plaque-reduction assay for chemosensitivity testing of human cytomegalovirus (HCMV) strains was developed based on early detection of viral plaques 96 h p.i. by a monoclonal antibody to the major immediate-early protein p72. Sequential HCMV isolates from an AIDS patient undergoing multiple courses of ganciclovir treatment during an 18-month follow-up were tested by the new assay, showing emergence of a ganciclovir-resistant strain. However, cloning of viral isolates and Southern blot hybridization analysis showed the simultaneous presence of three different HCMV strains in blood. Of these, the resistant strain was likely to be selected during prolonged maintenance antiviral treatment, emerging during full drug regimen, while the two sensitive strains reappeared in association with the resistant one following drug discontinuation. This finding was demonstrated by high levels of ID90 and ID99 in sequential mixed viral populations. The new plaque assay leads to reduction in time needed for chemosensitivity testing and permits rapid tracing of drug-resistant strains in a mixed viral population.