Antibodies against Epstein-Barr virus, associated with antibody dependent cytotoxic cell activity, were found to be present in diminished titer in 20 of 22 patients tested with chronic mononucleosis syndrome (CMS). Gamma globulin was shown to improve symptoms in 53% of the patients treated, compared with 32% of placebo injections. 89.5% of 57 patients treated with a gamma globulin treatment program remained in the treatment program because of relief of symptoms, and only four patients dropped out because there was no relief of symptoms or side effects. Four patients experienced complete relief of symptoms following a variable length treatment program. It would appear that intramuscular gamma globulin treatment is efficacious in the treatment of CMS and that the average interval between treatments is three weeks.