Mononuclear cell infiltrates are found to varying degrees in 30% to 60% of primary human central nervous system (CNS) gliomas. To explore the immunological importance of this, six operative glial tumors, eight non-glial tumors, and three normal brain specimens were studied. Utilizing an immunoperoxidase method, the authors examined frozen sections for lymphoid infiltrates expressing suppressor/cytotoxic and helper phenotypes, as identified with the Leu-1,2,3 monoclonal antibodies. Four of six gliomas demonstrated lymphoid infiltrates: three tumors exhibited a predominant suppressor/cytotoxic cell phenotype and the fourth showed mixed staining of suppressor/cytotoxic and helper cell phenotypes. Varying degrees of lymphoid infiltration characterized four out of eight non-glial primary CNS tumors. Two cases exhibited a prevalence of suppressor/cytotoxic phenotype cells, while two cases demonstrated a more heterogeneous pattern of phenotype expression. Normal brain sections revealed little or no evidence of mononuclear infiltrates. The immunobiological significance of these findings is discussed in the context of tumor-host interaction within the CNS.