Studies designed to identify a panel of monoclonal antibodies useful for the separation of murine eosinophil myeloid progenitors revealed that F4/80, an antigen heretofore thought to be expressed only by murine monocyte/macrophage lineage cells, was expressed by eosinophil granulocytes. Eosinophils from several strains of mice stained positively with specific antibody for the epitope. A novel pathway for myeloid differentiation is proposed in which neutrophil progenitors exit a common lineage prior to a common progenitor of monocytes and eosinophils. Moreover, the results demonstrate that binding of anti-F4/80 can no longer be viewed as exclusive for mononuclear phagocytes.