INTERFERON was discovered1 because of its antiviral properties, but interferon preparations have other biological activities. These include inhibition of transplanted tumours in vivo2 and of cell multiplication in vitro3,4, as well as regulatory roles in the immune response5. The effects of interferon on immune responses in vivo and in vitro have mostly been known as inhibitory, in particular on B-cell functions, whereas we have discovered augmented killer T-cell generation in vitro in the presence of interferon6. As interferon is produced in vivo in the course of acute viral infections (or after vaccination) at a stage when the temperature of the host is elevated7–9 we have investigated the influence of increased temperature on some of the effects of interferon. We report here that elevated incubator temperatures potentiate the antiviral activity, the killer-augmentory properties and the growth-inhibitory action of interferon on a lymphoblastoid cell line (Daudi).