We report herein the effect of soluble CD23 (sCD23) on the differentiation of early lymphoid and myeloid human precursors. CD23 is known as a low affinity receptor for IgE. In addition, our results show that sCD23 in synergy with IL1 has a potent activity on the maturation of prothymocytes and the proliferation of multipotential normal and leukemic myeloid precursors.