Ganciclovir susceptibility studies were done on the last cytomegalovirus (CMV) isolate available from 42 solid organ transplant recipients with CMV viremia who were enrolled in a prospective CMV prophylactic trial. Viruses were categorized as follows: 13 isolates (group 1) from patients who received ganciclovir prophylaxis; 9 isolates (group 2) from patients who received acyclovir prophylaxis; 8 isolates (group 3) from patients who received acyclovir prophylaxis and ganciclovir treatment; and 12 isolates (group 4) from patients who received prophylaxis and treatment with ganciclovir. All CMV isolates were sensitive to ganciclovir (mean 50% inhibitory concentration [IC50] 1.7 microM; range, 0.2-5.3 microM). Mean IC50 values (in microM) were 1.7 for group 1 isolates, 1.2 for group 2 isolates, 2.2 for group 3 isolates, and 1.7 for group 4 isolates (P > .05). Thus, acyclovir or ganciclovir prophylaxis in solid organ transplant patients did not select ganciclovir-resistant isolates of CMV.