The percentage of T lymphocytes that form rosettes (RFC) with sheep erythro-cytes (E rosettes), cutaneous delayed-type hypersensitivity, and in vitro lympho-cyte transformation was measured in 15 patients with primary intracranial tumors. The ability of T lymphocytes to form E rosettes was significantly impaired. These patients also exhibited impaired ability to become sensitized to dinitrochlorobenzene and humoral suppression of PHA-induced lymphocyte blastogenesis. The hypothetical role of antibody(s) to possible shared antigenic determinants on brain and T cells is discussed.