Using the peripheral blood lymphocyte (PBL) interferon induction test, impaired production of gamma interferon (IFN) was investigated in patients with malignant gliomas. 10⁷ human PBLs were placed in 5 cm diameter plastic dishes containing 5 ml of RPMI-1640 medium with 10% heat-inactivated fetal calf serum (FCS) and either 5 μg/ml of OK-432 or 10 units/ml of recombinant interleukin-2 (rIL-2) for 72 hours at 37°C. The culture supernatant was then collected and used for IFN assay which was carried out in FL cells with Sindbis virus. The yields of human IFN were expressed as laboratory units; one laboratory unit corresponds to the quantity necessary to cause 50% inhibition of the viral cytopathic effect. IFN production from PBL of healthy subjects, induced by OK-432 and rIL-2, was 5, 367±525 and 344±108 units/ml, respectively. The production from patients, however, was 198±66 and less than 40 units/ml, respectively. Human lymphokineactivated killer (LAK) cells were generated by placing 2.5 × 10⁶ PBL/ml in complete medium with 10% heat-inactivated FCS and either 5 μg/ml of OK-432 or 10 units/ml of rIL-2 for 72 to 96 hours at 37°C. The antitumor cytotoxic activity was estimated in triplicate by 4 hours ⁵¹Cr release assays. The in vitro cytotoxicity of LAK cells generated from PBL of healthy subjects by rIL-2 and OK-432 against allogeneic glioma (ONS-12) cells was 75% and 50%, respectively. The LAK activity induced in patients by rIL-2 and OK-432 was between 55 and 30%, and between 30 and 7%, respectively. It was noted that very little antitumor activity (i.e. 7%) was induced by OK-432 in one patient with an initial low IFN production. The cytotoxic activity of LAK cells induced by OK-432 correlated to IFN production induced from PBL of patients with gliomas.