Thirteen patients severely infected with herpesvirus were treated with intravenous adenine arabinoside (ara-A), and two patients received placebo therapy. Blastogenic and cytotoxic responses specific for the virus infecting each patient were determined before, during, and after treatment. Blastogenic responses to three mitogens (phytohemagglutinin, pokeweed, and concanavalin-A) were examined, as were titers of viral antibody. In vitro responses during and after treatment with ara-A were unchanged or often enhanced as compared with values before treatment. Newborn infants, presumably infected at or shortly before birth, did not demonstrate cellular immune reactivity to the infecting virus until after four days of life. In this series of patients, prognosis appeared to be in part determined by the cellular immune competence of the host at the time of infection.