Mesothelioma is a malignant tumor of the serous surfaces in the thorax and abdomen, which has proved exceptionally resistant to treatment. A recent phase II trial of a high-dose methotrexate regime on 63 Norwegian patients has, however, achieved a response rate of 37%. Some responses have also been achieved using interferon (IFN)-γ administered intrapleurally or recombinant (r) IFN-α administered subcutaneously. Our earlierin vitrosensitivity studies of mesothelioma cell lines showed that IFN augments the response to chemotherapeutic agents in mesothelioma. The aim of this study was to assess the response of four mesothelioma cell lines, derived from diffuse asbestos-related pleural malignant mesothelioma, to methotrexate alone and in combination with recombinant IFN-α and IFN-γ. Anti-proliferative effects were assayed by vital dye exclusion. A combination of IFN-α and IFN-γ consistently augmented the response of the cell lines to methotrexate, by as much as 75% for one cell line, although the response to the individual IFNs was variable. We were also able to compare the effects of natural IFN-β with those of IFN-α and IFN-γ. The IFN-β sensitivity profile for each of the four cell lines was similar to that of IFN-α. In two cell lines, the combination of IFN-β and IFN-γ produced a similar effect to the IFN-α and IFN-γ combination.