Several studies including ours have suggested that lack of CD56 expression in multiple myeloma (MM) defines a unique patient subset with poorer prognosis. However, the mechanism underlying this aggressive behavior of CD56− MM has not been well elucidated. In this study, we sorted out both CD56− and CD56+ fractions from MM cell lines or patients with MM, and investigated their different responsiveness to interleukin-6 (IL-6) or insulin like growth factor-I (IGF-I), and tried to clarify the course of action in cell cycle distribution. After stained with PE-CD56, CD56− and CD56+ fractions in KMS-21-BM and U-266 cell lines were isolated by the cell sorter, and cultured separately either in the presence or absence of IL-6 (2 ng/ml and 10 ng/ml, respectively). Although CD56− cells in both KMS-21-BM and U-266 cell lines responded significantly to IL-6 (P=0.001 and 0.009, respectively), CD56+ cells did not. Ki-67+ cells in CD56− KMS-21-BM cells, that were significantly fewer than that in CD56+ ones (P=0.0003), increased significantly upon 24-hour incubation with IL-6 (P<0.0001). Western blotting analysis showed that the level of cyclin D1 and p27 protein in CD56− KMS-21-BM cells were up- and down-regulated by IL-6 in a time dependent manner, respectively. IL-6 also brought phosphorylation of Akt (ser473) in the CD56− cells. LY-294002 completely blocked these effects of IL-6. On the other hand, Ki-67+ cells in the CD56+ cells did not respond to IL-6. Although IGF-I did not increase Ki-67+ cells either in the CD56− and CD56+ cells from KMS-21-BM, anti-IGF-I mAb significantly reduced Ki-67+ cells only in the CD56+ cells (P=0.006). IGF-I up-regulated the level of cyclin D1 and phosphorylated Akt in CD56+ KMS-21-BM cells. LY294002 completely blocked these effects of IGF-I. Same results were obtained in the analysis of U-266 cell lines. The MM cells sorted from 17 patients with MM were also examined for CD56 and Ki-67 expression. Four and 13 patients were distributed to the CD56− and CD56+ group, respectively. These MM cells from the patients were cultured with or without IL-6 (10 ng/ml) or IGF-I (500ng/ml) for 24 hours. IL-6 increased the percentage of Ki-67+ cells in the CD56− group more than those in the CD56+ group (P=0.007). Although MM cells did not respond to IL-6, IGF-I significantly increased Ki-67+ cells in the CD56+ group (P=0.005). These results suggest that CD56− and CD56+ MM cells could be stimulated by different cytokines. We here found that CD56− MM cells were proliferated more than CD56+ MM cells in the presence of IL-6, and that this effect of IL-6 was mainly mediated by the activation of PI3-K/Akt pathway. In addition, our results suggest that IGF-I play an important role in the proliferation of CD56+ MM cells via PI3-K/Akt pathway.