The genetic sequences encoding the gibbon and human interleukin 3 (IL 3) proteins were molecularly cloned. The amino acid sequence of the mature gibbon IL 3 protein proved to share 93% homology with the corresponding human protein. We examined the effects of biosynthetic (recombinant) gibbon IL 3 on the proliferation and differentiation of an enriched population of human hematopoietic progenitors and compared the results with the effects of recombinant human granulocyte-macrophage colony-stimulating factor (GM-CSF). Gibbon IL 3 supported the formation of various types of single lineage as well as multilineage colonies by My-10+ bone marrow cells in the presence of human erythropoietin (Ep). In contrast, recombinant human GM-CSF supported the formation of single-lineage colonies and only a small number of multilineage colonies. Both IL 3 and GM-CSF had significant erythroid burst-promoting activity (BPA). Delayed addition of gibbon IL 3 to low serum culture of My-10+ marrow cells supported the formation of blast cell colonies with variable but high replating capability. Human GM-CSF was less effective than IL 3 in support of multipotential blast cell colonies. These results are analogous to the effects of murine IL 3 and GM-CSF on murine progenitors and support the notion that the primary factor for multipotential progenitors is IL 3.