A 66-year-old Japanese man developed severe anemia and erythroid hypoplasia in bone marrow without any significant underlying disease. The results of an antiglobulin test were strongly positive, and serum erythropoietin (Epo) was high. The patient was diagnosed as having acquired pure red cell aplasia (PRCA) and was treated with steroids. Anemia was subsided by reticulocyte production in parallel with a decrease in the titer of antiglobulin test and the level of Epo. We studied the immunological mechanism directed against erythroid cells in vitro by using the patient's serum. In vitro analysis indicated the presence of an inhibitor of erythroid precursors at onset, and its disappearance at remission, suggesting the presence of inhibitor against erythroid precursors.