The tumor necrosis factor family member CD70 (CD27L) is an integral membrane protein transiently expressed on antigen-activated T and B lymphocytes. CD70 regulates lymphocyte survival and differentiation of CD27 positive resting T cells and antigen-primed B cells. Aberrant CD70 expression has been detected on hematologic tumors including diffuse large cell, follicular, and mantle cell lymphomas, chronic lymphocytic leukemia and Reed-Sternberg cells in Hodgkin’s disease. Here we report for the first time the expression of CD70 on multiple myeloma (MM). Nine of thirteen MM cell lines examined expressed detectable surface CD70 with receptor copy numbers ranging from 9,000 to 145,000. Furthermore, seven of twelve primary MM isolates and six of six Waldenstrom’s macroglobulinemia (WM) isolates tested were positive for CD70. The presence of CD70 on MM and WM, combined with the antigen’s limited expression on normal non-hematopoietic tissues makes CD70 an attractive target for antibody-directed therapy of plasma cell disorders. We have engineered chimeric and humanized variants (human IgG1 isotype) of the murine anti-CD70 monoclonal antibody, 1F6. Both chimeric and humanized 1F6 variants retain the binding specificity of the murine parent monoclonal antibody (mAb) and acquired Fc-dependent effector functions including antibody-dependent cellular cytotoxicity (ADCC), complement dependent cytoxicity (CDC) and macrophage-mediated antibody-dependent cellular phagocytic (ADCP) activities against CD70+ tumor cells. Specific lysis (ADCC, CDC) or tumor cell uptake (ADCP) was observed when engineered anti-CD70 mAb were used at ≥10 ng/mL against a panel of CD70-expressing hematologic tumors. The significance of these in vitro observations was further established by the ability of anti-CD70 mAb to significantly improve the survival of mice bearing disseminated CD70-expressing tumors. The in vitro function and in vivo efficacy of anti-CD70 were dependent upon Fc-FcγR interactions, as activity and survival benefit was lost with anti-FcγR blockade or when the targeting antibody was engineered to eliminate FcγR binding. Our data suggest that humanized anti-CD70 mAb possesses potent antitumor activity and has potential utility for therapy of hematologic cancers including MM and WM.