Abstract:The influence of interferon (IFN) on antibody production and viability of malignant cells from patients with multiple myeloma was evaluated. Following incubation of bone marrow cells with IFN‐α (5000 units/ml) for 7 days) a decreased production of monoclonal immunoglobin (mlg) was detected in all experiments except one. IFN induced a>50% decrease in myeloma cell viability in 11 patients and a ≥ 25 % decrease in 4 patients. whereas in myeloma cells from 8 patients IFN had no or only minor effects. The observed effect was not due to an inhibition of proliferation since<5% of the myeloma cells were labeled with [3H]‐thymidine during 7 d of culture. There was no statistically significant correlation between decreases in myeloma cell viability and effects on mlg production, exemplified by the fact that mlg production was decreased also in patients showing no sensitivity to IFN's cytotoxic action. Depletion of autologus T‐cells, NK‐cells and/or monocytes did not abrogate the effects observed. We conclude that IFN can reduce the viability of myeloma cells and the production of Ig from these cells and that the latter can be exerted without an antitu