A technique for determining the susceptibility to ganciclovir of cytomegalovirus (CMV) strains isolated in clinical samples is described. The inoculum was composed of a partially infected suspension of cells from a young positive culture (10 days), usually the first passage of the primary culture. The appropriate dilution of the cell suspension to provide a suitable inoculum was based on a previous study of five strains grown in different dilutions which provided a countable number of plaques and avoided titration of each of the isolated strains. Fifty-three strains were studied at three different dilutions. Five from patients on maintenance ganciclovir therapy with poor clinical response had a 50% inhibitory dose (ID50) between 21.46 and 13.35 microM and the remainder an ID50 between 2.31 and 10.5 microM, comparable to results obtained by other authors using susceptibility techniques with a sonicated inoculum. Three of these strains were studied by both methods using sonicated inoculum and cell suspension inoculum. The mean time which elapsed between seeding the specimen and obtaining sensitivity was 39.00 and 27.66 days, respectively. The technique reduces significantly the time involved since relatively young cultures can be studied and previous titration is not required.
