Patients with primary malignant brain tumors manifest a variety of abnormalities in cell-mediated and humoral immunity. Diminished T cell reactivity has been shown in these patients to be linked to deficiencies in interleukin 2 (IL-2) production that cannot be overcome by exogenous IL-2. In this study, specific binding of radiolabeled IL-2 to PHA-stimulated lymphocytes from brain tumor patients demonstrates that the number of high affinity interleukin 2 receptors (IL-2R) is greatly reduced. FACS analysis indicates that the relative density of the p55 protein (Tac protein) is lower on the mitogen-activated lymphocytes obtained from patients than on comparably treated lymphocytes from normal individuals. These data indicate that mitogen-stimulated lymphocytes obtained from patients have fewer functional high affinity IL-2R principally because of the failure to express sufficient levels of the p55 protein for association with the p75 protein. Northern analysis of total RNA isolated from mitogen-stimulated T cells from patients demonstrates normal levels of steady state mRNA, which codes for the p55 protein. Moreover, there is no defect in the postranslational processing of the primary translation product of this mRNA suggesting that normal levels of the p55 protein are produced in activated T cells from patients.