Interferon was originally described as an antiviral agent produced shortly after onset of infection with most viruses. However, in addition to inducing an antiviral state, interferon inhibits cell division, increases the expression of cell-surface antigens, boosts the cytotoxic activity of natural killer (NK) cells and modulates several immune functions of lymphocytes and macrophages. Moreover, a special class of interferon (immune interferon or IFN-gamma) is produced by T cells following stimulation with antigen or interaction with mitogens. The different methods by which interferon is induced and its multiple effects suggest that it may be part of a first-line defence system controlling the spread of virus infections and the proliferation of modified 'self' cells that have been affected by virus infection or neoplastic transformation. The ability of certain human lymphoma cells to activate the alternative pathway of complement is well established. Here we show that monoclonal antibody-purified interferon can amplify the ability of certain tumour cells to activate complement via the alternative pathway. This demonstration may reflect an additional, as yet unknown, role of interferon in inducing non-specific anti-tumour immunity.