To investigate the therapeutic activity of the fully human anti–HLA-DR antibody 1D09C3 in multiple myeloma (MM), we reevaluated HLA-DR expression on CD138+ cells, analyzed the capacity of IFN- to up-regulate HLA-DR expression on MM cell lines, and tested the in vitro and in vivo activity of 1D09C3 alone or in combination with IFN-. CD138+HLA-DR+ cells were detected in 31 of 60 patients, with 15 of 60 patients having 20% CD138+HLA-DR+ cells (median, 50%; range, 23–100). Because primary plasma cells cannot be efficiently cultured in vitro, we used a panel of MM cell lines with a dim/negative to bright HLA-DR expression to evaluate 1D09C3-induced cell death. Annexin V/propidium iodide (PI) staining showed that 1D09C3-induced cell death correlated with constitutive HLA-DR expression. Induction of HLA-DR by IFN- restored the sensitivity of HLA-DR dim cell lines to 1D09C3. In vivo, the combined IFN-/1D09C3 treatment significantly increased the median survival of nonobese diabetic/severe combined immunodeficient mice xenografted with KMS-11 cell line, compared with controls (147 versus 48 days, P 0.0001) or mice receiving 1D09C3 alone (147 versus 92 days, P 0.03). The better therapeutic activity of IFN-/1D09C3 treatment over 1D09C3 alone was further shown by a 2-fold increase of mice being disease-free at 150 days after xenograft (47% versus 25%). No mice experienced any apparent treatment-related toxicity. Our data show that (a) one fourth of MM patients express HLA-DR on CD138+ cells and (b) IFN-–induced up-regulation of HLA-DR results in a potent enhancement of the in vivo antimyeloma activity of 1D09C3.