Human cell culture-derived interferon was shown to inhibit human cytomegalovirus in vitro. A prototype strain, Davis, and six clinical isolates of cytomegalovirus were tested. All six isolates showed uniform susceptibility to interferon, exceeding that of the Davis strain by two-to fourfold. The latter virus was found to be 32 to 4 times less susceptible than the sensitive indicator, vesicular stomatitis virus. However, the laboratory finding of susceptibility to an antiviral material may not relate to its clinical effectiveness.Human cytomegalovirus (CMV) causes a broad spectrum of clinical illness (21). The devastating effects of congenital disease have long been recognized. More recently, subtle sequelae, including decreased mentation and auditory impairment, have been described, even among children with subclinical presentation at birth (18). Although acquired CMV infection is generally benign, significant morbidity and mortality occurs, especially in organ transplant recipients and immunosuppressed patients (16,20; S. Suwansirikul, N. Rao, J. N. Dowling, and M. Ho, Arch. Intern. Med., in press).Therapeutic trials have been attempted with a number of agents. CMV is inhibited in cell culture by the nucleoside analogues, 5-iodo-2'-deoxyuridine, adenine arabinoside, and cytosine arabinoside (7). In practice, these compounds may transiently suppress urinary excretion of CMV, but generally fail to eradicate the virus or alter the natural course of the disease (4, 5,13,15,19). Administration of massive interferon doses may also affect CMV excretion in the urine. Viruria was completely inhibited in three ofnine patients with congenital or acquired infection treated by Emodi et al. These results are not surprising in view of the relative nonsusceptibility of CMV to interferon in cell culture. Early work by Glasgow et al. (8) demonstrated that CMV is more resistant to the antiviral action of interferon than other viruses, including Sindbis, vaccinia, and vesicular stomatitis virus (VSV), to which it was compared. However, the amount of interferon employed was relatively small. The antiviral effect of interferon against CMV needs to be reassessed in view of the massive doses now available for human administration. In this study, the in vitro inhibition of CMV by human interferon was characterized quantitatively.The antiviral effect on the Davis strain of CMV was compared with that observed with VSV. In addition, the susceptibility of six clinical isolates of CMV was determined.