A patient with pure red cell aplasia (PRCA), who had the inhibitor to erythroid precursors in serum, is described. A 72-year-old female was referred to Nagoya National Hospital because of progressing anemia in April 1988. On admission, her hemoglobin was 4.8 g/dl, reticulocyte 0.8%, and bone marrow specimen contained only 1.2% erythroblast. On these bases, she was diagnosed as pure red cell aplasia. After small amount of blood was transfused, her hemoglobin and erythroblast in bone marrow (EBM) increased to 7.8 g/dl and 39.1%, respectively, and she was discharged. However, after a month, her hemoglobin dropped to 4.6 g/dl, reticulocyte to 0.1%, and EBM to 0%. Soon after corticosteroid therapy (prednisolone, 40 mg, daily) was started, a marked elevation of reticulocyte count was observed, and then her hemoglobin increased to 11.0 g/dl, and EBM to 31.6%. The reason for a transient spontaneous remission at the onset of her disease was occurred is unclear. The number of BFU-E in her bone marrow was within normal range, but it was suppressed significantly (65%) after the addition of her serum and the complement purified from rabbit plasma. This finding suggest the presence of inhibitor to erythroid precursors in her serum.