Different multiple myeloma cell lines have different levels of sensitivity to proteasome inhibitors. Five different human multiple myeloma cell lines were cultured for 12 (A) and 24 (B) hours in increasing concentrations of the proteasome inhibitors epoxomycin (EPOX), PSI, and Bortezomib (BZ). Cell viability was determined by flow cytometry following Annexin V–FITC and propidium iodide staining. The data are presented as the mean ± SD from at least 3 independent experiments.