Chronic exposure of humans to benzene (BZ) affects hematopoietic progenitor cells in intermediate stages of differentiation which can lead to aplastic anemia and/or acute myelogenous leukemia and some of its variant forms. We studied the effects of BZ and hydroquinone (HQ), a toxic bone marrow metabolite, on the human HL-60 promyelocytic leukemic cell line. Because the HL-60 cell is bipotential and can be induced to differentiate to monocytes or granulocytes it has been used in many studies as a surrogate for the granulocyte/macrophage committed cell, GM-CFU. Treatment of HL-60 cells with BZ specifically induced differentiation along the granulocytic lineage as measured by morphology, induction of superoxide production and chloroacetate esterase activity and the appearance of the LI2–2 surface antigen. Differentiation was induced via the activation of protein kinase C and the phosphory-lation of several proteins known to be involved in HL-60 cell differentiation. Subsequent to kinase C activation, arachidonic acid was released from membrane phospholipids and the 5-lipoxygenase pathway was activated for the production of leukotriene D4(LTD4) required for granulocytic differentiation. BZ induction of granulopoiesis was prevented by preincubation of HL-60 cells with inhibitors of protein kinase C, 5-lipoxygenase,γ-glutamyl transpeptidase required for the conversion of LTC4to LTD4, or LTD4receptor antagonists. Treatment of HL-60 cells with tetraphorbol myristate acetate (TPA), lα, 25-dihydroxyvitamin D3(1,25-(OH2)D3) or interleukin-1 (IL-1) induced HL-60 cells to differentiate to monocytes/macro-phages. Hydroquinone prevented induction to monocytes/macrophages induced by TPA or lα,25-dihydroxyvitamin D3, but not by IL-1 and significantly, had no effect on the induction of granulocytic differentiation by BZ or any other inducer. A mechanism for the induction of granulocytic differentiation by BZ is proposed.