We studied the expression dynamics of inhibitor of apoptosis protein (IAP) family members and Smac/DIABLO after treatment with doxorubicin in human multiple myeloma cell line RPMI 8226 and its doxorubicin-resistant variant DRR. Proapoptotic stimulation with doxorubicin rapidly induced the overexpression of mRNA as well as protein for IAPs in RPMI 8226 cells followed by a gradual decrease of their expression. Smac/DIABLO, which is known to neutralize IAPs, showed increased expression at the mRNA level after treatment; however, Western blot analysis revealed a slight decrease of the amount of protein. Immunoprecipitation analysis revealed the association of Smac/DIABLO with cIAP1 or XIAP after treatment with doxorubicin. In contrast to the RPMI 8226 cells, DRR cells did not undergo apoptosis in response to doxorubicin treatment. The DRR cells had higher levels of IAPs expression at the mRNA level and did not show a remarkable peak or decrease in the expression of mRNAs for cIAP1, cIAP2, XIAP, and survivin after treatment with doxorubicin. Furthermore, the expression of Smac/DIABLO mRNA was not up-regulated after treatment. These findings indicate that the suppression of IAPs expression by Smac/DIABLO shortly after proapoptotic stimulation might play a role in the mechanisms of apoptotic induction, and that the maintenance of high IAPs expression and low Smac/DIABLO expression after treatment might lead to the doxorubicin-resistance of multiple myeloma cells.