A rapid screening assay for the detection of resistance to ganciclovir and foscarnet of primary isolates of human cytomegalovirus from culture-positive blood samples was developed by using single doses of both drugs and an immediate-early antigen plaque reduction assay. Results of the rapid assay with peripheral blood leukocytes as the viral inoculum were compared with those of a conventional assay with cell-free virus from the relevant viral isolates. Both assays gave overlapping results. The rapid assay offers the following major advantages: (i) it provides results within 4 to 6 days, (ii) it can be performed with peripheral blood leukocytes, and (iii) it reliably detects drug-resistant human cytomegalovirus strains.