The primary objectives of the study are to evaluate the safety profile and the anti-tumor activity of 2 dose levels of natalizumab in participants with relapsed or refractory multiple myeloma. Secondary objectives are to assess the pharmacokinetic (PK) profile of natalizumab in this study population and to assess peripheral blood mononuclear cell (PBMC) saturation of very late antigen-4 (VLA-4, an α4-integrin) and evaluate possible correlations with clinical activity.