We present evidence for the continuous erythroid differentiation of a cell line, KU-812-F, without the addition of an inducer. Erythroid differentiation of these cells was confirmed according to the following findings: 1) erythroid morphology; 2) existence of glycophorin A and carbonic anhydrase I as a membrane marker protein and an enzyme characteristic of erythroid cells, respectively; 3) synthesis of adult type (HbA and HbA2) and fetal type (HbF) hemoglobins, as detected on isoelectric focusing; and 4) transcription of the alpha-, beta-, and gamma-globin genes, as detected on Northern blot analysis.