Glioblastoma cells release factors (G-TsF) which inhibit T-cell proliferation. The G-TsF is a novel member of the transforming growth factor beta family and is identical to TGF beta 2. The effect of G-TsF and TGF beta 2 on the induction of LAK cell activity was investigated by culturing PBL obtained from normal blood donors and brain tumour patients in varying concentrations (50-500 U/ml) of interleukin 2 (IL2) alone or IL2 plus G-TsF/TGF beta 2 (1 ng/ml) for 4 days. Subsequent cytolytic activity was measured against autologous and allogeneic glioblastoma targets, fresh NK-resistant melanoma cells and K562 cells. G-TsF/TGF beta 2 purified from glioblastoma cell cultures and TGF beta 2 isolated from porcine platelets significantly suppressed the generation of LAK cell activity, and the inhibitory effect could be reduced by higher concentrations of IL2. The suppressive effect of TGF beta 2 was most significant during the early stages of LAK cell generation and no inhibitory effect was seen when TGF beta 2 was added directly to the cytotoxicity assay. These results suggest that human glioblastomas may exert an inhibitory influence on the generation of an immune response in vivo through the production of G-TsF/TGF beta 2, and that the inhibitory effect may be modified by IL2.