This report describes the first of 2 investigations studying mechanisms of Epstein-Barr virus (EBV) persistence in the infected host; specifically, we wish to determine the extent to which virus carriage within the B-cell system is dependent upon continued replication of the virus in permissive oropharyngeal epithelium. Levels of EBV infection at these 2 sites have been monitored in 21 acute infectious mononucleosis (IM) patients before, during and after treatment with high doses of acyclovir (ACV). Twelve patients received oral ACV for 10 days and 9 patients received i.v. ACV for 5 days before the 10-day oral course; all were followed prospectively for 28 days. Infectious EBV, detectable at high initial levels in the patients' throat washings, disappeared almost completely during ACV treatment, then returned again to high levels post treatment. In contrast, levels of virus-infected B cells in the blood showed no reduction linked to the period of ACV treatment nor any increase with resumption of EBV shedding. During IM, therefore, maintenance of high levels of virus carriage within the B-cell pool is not dependent upon the continual recruitment of newly infected B cells. This might reflect an inability of the immune T-cell response in acute IM patients to prevent continued expansion of the existing EBV-infected B-cell pool. Alternatively, it raises the possibility that EBV carriage in B cells in vivo is maintained through a virus:cell interaction which is not sensitive to virus-specific T-cell surveillance.