A non-complement-dependent antibody that inhibits bone marrow proliferation in vitro was detected in the serum of a patient with systemic lupus erythematosus and aplastic anemia. The patient recovered from the aplastic anemia following treatment with corticosteroids and plasmapheresis. The antibody could no longer be detected after this recovery. This case demonstrates that a rare, but potentially lethal, complication of systemic lupus erythematosus is aplastic anemia due to an autoantibody. Results of the experiments suggest a mechanism for, and treatment of, similar cases.