Two patients with subacute sclerosing panencephalitis (SSPE) showed impairment of cell-mediated immunity, as indicated by a low T [thymus-derived] cell number, decreased intracellular cyclic AMP levels of peripheral blood lymphocytes, negative graft vs. host reaction in vivo, negative skin reactions to common antigens and, in 1 of the patients, abnormal reactions in migration inhibition factor tests. Since some of the impaired T cell functions in 1 of the patients were reconstituted in vitro by the administration of thymus humoral factor (THF), a course of THF administration was given to both patients. In vitro and in vivo assays, which reflect T cell competence, were performed before and after a daily schedule of THF administration that lasted for 10 days in 1 patient and 21 days in the other. THF was apparently capable of reconstituting the impaired T cell functions in both patients after a short term of treatment.