Susceptibility testing of 68 cytomegalovirus (CMV) peripheral blood isolates to Ganciclovir (DHPG) and 11 blood isolates to Foscarnet (PFA), was performed on primary culture isolates using the shell vial assay methodology (SVA-IFA, that is, quantitation of fluorescent focus units, FFUs), with an anti-CMV monoclonal antibody to the late viral antigen. A positive reaction in monolayer cultures of MRC-5 cells was characterized by cytoplasmic fluorescence with inclusions at both or more commonly off one end of the elongated fibroblast nucleus. Isolates from conventional MRC-5 tube cultures displaying a 1 + (10% cytopathic effect) were inoculated into shell vials containing DHPG concentrations of 0, 1.5, 3, 6, 12, or 24 μl/ml or shell vials containing 400, 500, 800, or 1200 μM PFA. The optimal readability of monolayers (expressed as FFUs per monolayer) occurred at 96 h after treatment with DHPG and at 36–48 h with PFA. Resistance to DHPG was determined at the concentration of antiviral agent necessary to reduce the number of FFUs to 90% or 50% of the control [that is, the 90% minimum inhibitiory concentration (MIC 90) or MIC 50]. Six of 68 isolates showed an MIC 90 > 12 or an MIC 50 > 1.5 μg/ml, and were considered DHPG resistant. Three of the six isolates were from AIDS patients with latestage disease who had never received DHPG therapy. All but one (specimen 2400) DHPG-resistant isolates revealed MIC 90 values to a PFA concentration of 500 μM, which is considered an achievable peak plasma level in patients undergoing PFA therapy. The single DHPG- and FPA-resistant isolate was obtained from a patient displaying marked clinical resistance to both drugs. The use of a monoclonal antibody to the CMV late antigen enables a pattern to be easily recognized by SVA-IFA, enabling anti-CMV susceptibility testing to be completed within days after primary isolation.