As survival regulation is a key process in multiple myeloma biology, we have studied the Bcl-2 family proteins that can be regulated by three myeloma cell survival factors: interleukin-6 (IL-6), interferon-alpha (IFN-alpha) and insulin-like growth factor (IGF-1). Eleven myeloma cell lines, whose survival and proliferation are dependent on addition of IL-6, variably expressed 10 anti-apoptotic or pro-apoptotic proteins of the Bcl-2-family. When myeloma cells from four cell lines were IL-6 starved and activated with IL-6 or IFN-alpha, we observed that only Mcl-1 expression was up-regulated with myeloma cell survival induction. Nor was obvious regulation of these 10 pro-apoptotic or anti-apoptotic proteins found with IGF-1, another potent myeloma cell survival factor. Our results indicate that the myeloma cell survival activity of IL-6 linked to Bcl-xL regulation cannot be generalized and emphasize that Mcl-1 is the main target of IL-6 and IFN-alpha stimulation. However, other changes in the activity of the Bcl-2 protein family or other apoptosis regulators must be identified to elucidate the IGF-1 action mechanism. Cell Death and Differentiation (2000) 7, 1244 - 1252.