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Daratumumab is a humanized anti-CD38 IgG1 monoclonal antibody which could be used for multiple myeloma (MM). MM with plasma-cell leukemia (PCL) transformation is highly aggressive and is resistant to conventional therapy. Novel therapeutics are needed for PCL, and daratumumab may play role. We report a case of relapsed/refractory multiple myeloma (RRMM)-transformed PCL successfully treated with daratumumab. The case was a 42-year-old man who was diagnosed with MM 2 years ago and relapsed after six cycles of bortezomib-based chemotherapy. The patient rapidly developed hyperleukocytosis and disseminated intravascular coagulation, and was diagnosed with PCL. Daratumumab-based therapy was tried and the case miraculously obtained complete remission (CR) after four doses of a weekly infusion of daratumumab. Finally the patient received autologous hematopoietic stem-cell transplantation (auto-HSCT) and maintained CR. Moreover, we monitored the immune cell dynamics by flow cytometry (FCM) during daratumumab-based treatment. The immune cell subset analysis revealed significant down-regulation of CD38+ natural killer (NK) cells, regulatory T cells (Tregs) and regulatory B cells (Bregs). Meanwhile cytotoxic T-lymphocyte expansion was observed. In conclusion, daratumumab could rapidly decrease tumor burden, improve the condition of the PCL patient, and serve as a bridging salvage chemotherapy for further chimeric antigen recptor T cell therapy (Car-T) or HSCT, which could potentially improve patient survival. The immune cell dynamic findings in this case suggest that the immunomodulatory mechanism may contribute to the antimyeloma effect of daratumumab., Competing Interests: Conflict of interest: The author(s) declare that there is no conflict of interest., (© The Author(s), 2021.)