Background: Since multiple myeloma responds poorly to conventional chemotherapy or radiotherapy, new therapeutic approaches are needed. This study investigated the effects of dexamethasone, all-trans retinoic acid (ATRA), the active metabolite of vitamin D-3 [1,25(OH)(2)D-3] and interferon-alpha on FO mouse myeloma cells (non-immunoglobulin-secreting myeloma cell line) in single drug or drug combination groups in vitro. Methods: Apoptosis ratio and change in cell counts in 4 single drug groups (dexamethasone, ATRA, vitamin D-3 and interferon-alpha) and 6 combination drug groups (dexamethasone + vitamin D-3, dexamethasone + ATRA, dexamethasone + interferon-alpha, vitamin D-3 + ATRA, vitamin D-3 + interferon-alpha, interferon-alpha + ATRA) were compared with the control group. Results: When treatment groups were compared with the control group, there was a significant increase in apoptosis in all, but this was most prominent in the group treated with dexamethasone alone. The apoptosis ratios were 0.10 and 6.82% in the control and dexamethasone-only groups, respectively. We also found that there was a significant decrease in cell count, particularly in the dexamethasone-only, ATRA-only, and ATRA-vitamin D-3 combination groups. Conclusion: ATRA, interferon-alpha, vitaminD(3) and particularly dexamethasone have significant effects on FO mouse myeloma cells resulting in a decreased cell count and an increased apoptosis ratio. This study should be repeated with human myeloma cell lines for further information. Copyright (C) 2004 S. Karger AG, Basel.