There is considerable interest in identifying nontoxic differentiation inducers for the treatment of various malignant and nonmalignant blood disorders, including inborn beta-chain hemoglobinopathies. Using the human leukemic K562 cell line as a model, we explored the efficacy of phenylacetate, an amino acid derivative with a low toxicity index when administered to humans. Treatment of K562 cultures with pharmacologically attainable concentrations of phenylacetate resulted in erythroid differentiation, evident by the reduced growth rate and increased hemoglobin production. The effect was time- and dose-dependent, further augmented by glutamine starvation (phenylacetate is known to deplete circulating glutamine in vivo), and reversible upon cessation of treatment. Molecular analysis showed that phenylacetate induced gamma globin gene expression with subsequent accumulation of the fetal form of hemoglobin (HbF). Interestingly, the addition of phenylacetate to antitumor agents of clinical interest, eg, hydroxyurea and 5-azacytidine, caused superinduction of HbF biosynthesis. The results suggest that phenylacetate, used alone or in combination with other drugs, might offer a safe and effective new approach to treatment of some hematopoietic neoplasms and severe hemoglobinopathies.