Five high-grade (3 grade III and 2 grade IV) astrocytoma tumour cell populations were treated with a preparation of Human Leukocyte Interferon either in monolayer cell culture or as multicellular spheroids in vitro or as xenografts growing in immune-deprived mice in vivo. A moderate and transient sensitivity was seen in one grade III tumour when tested in both of the in vitro assays, but no inhibition of growth was seen in vivo. Two tumours which were apparently resistant to Interferon treatment responded to orthodox chemotherapy. When used in conjunction with BCNU, Interferon was not effective in prolonging delay in tumour growth. It is concluded that Interferon is unlikely to be an effective agent in the treatment of malignant brain tumours.