Since interferon administration in the treatment of some malignant conditions has been reported to cause leukopenia, we studied the effects of human leukocyte interferon preparation (HLIF) on myelopoiesis in vitro. The continual presence of HLIF in semisolid agar cultures caused a progressive decline in colony and a rise in cluster incidence with increasing concentrations of interferon. The total plating efficiency, however, remained nearly constant, regardless of the HLIF doses used. Morphological examination of the clusters demonstrated a progressively increasing percentage of immature granulocytic precursors with increasing HLIF concentrations. This suggests that HLIF causes leukopenia by blocking differentiation of marrow myeloid precursors.