Immunosuppressive events are often observed in glioblastomabearing patients. We tested the response of circulating lymphocytes from glioblastoma patients to low concentrations of interleukin (IL)-4 and IL-2 after lectin activation or specific in vitro stimulation by autologous tumor cells. In the presence of IL-2, IL-4 up-regulates the proliferation rate of phytohemagglutinin (PHA)-P-stimulated glioblastoma patients' peripheral blood lymphocytes (PBL)s. Allogeneically- and syngeneically-stimulated PBLs of these patients present an increased proliferation rate in the presence of IL-4. This specifically stimulated lymphocyte population presents a very low proportion of CD8+ cells. This proportion is slightly increased in the presence of IL-4. Our results indicate that the glioblastoma cell-imposed inhibition on T-cells can be partly overcome by low concentrations of IL-4 during in vitro stimulation. Our experiments also demonstrate that glioblastoma-bearing patients' PBLs constitute a good model in which to study the effects of IL-4.