T cells stimulated by mitogens¹, specific antigens², including viral antigens³, and alloantigens⁴ have been shown to produce immune Interferon (IFN-γ). However, the exact circumstances of induction of IFN-γ in T cells have not yet been established, particularly the precise nature of the cell type producing the Interferon. We have recently described⁵ the selection and cloning of a BALB/c cytotoxic T(T_c)-cell line (L4) which grows in the presence of T-cell growth factor (TCGF), is Lyt-2 positive and kills H-2^d target cells infected with any type A influenza vims. The aim of the present study was to examine whether this cloned cytotoxic T-cell line produced IFN-γ on contact with the appropriate target cell. It was found that small amounts of IFN-γ were produced on co-culture of L4 with target cells, whereas L4 cells on their own released no detectable Interferon. The pattern of antigenic specificity by L4 cells for interferon production resembled the requirements for killing in that T_c cell mediated lysis was restricted to H–2^d targets infected with influenza A, but not influenza virus B strains, and interferon production was not prevented by a monoclonal antibody to the virus' haemagglutinin, which does not inhibit cytotoxicity while neutralizing the virus.