Daratumumab, a human IgG1κ monoclonal antibody targeting CD38, is used to treat multiple myeloma. We describe successful treatment with daratumumab in a case of treatment-refractory pure red-cell aplasia after ABO-mismatched allogeneic stem-cell transplantation. The patient was a 72-year-old man with the myelodysplastic syndrome who received a transplant from an HLA-matched, unrelated donor with a major ABO incompatibility (blood group A in the donor and blood group O in the recipient). The patient had persistent circulating anti-A antibodies and no red-cell recovery 200 days after transplantation. Standard treatments had no effect. Within 1 week after the initiation of treatment with daratumumab, he no longer required transfusions.