The induction of cell differentiation and growth inhibition of HL-60 cells by Bc-4, an active principle from Boswellia carterii, was studied in vitro and in vivo. The proliferation of HL-60 cells was found to be inhibited by Bc-4 at a concentration of 5-10 micrograms/ml. Under the action of Bc-4, the acid phosphatase and NBT reduction activities in HL-60 cells were increased, and phagocytosis of cells was also induced. All these activities were concentration dependent. The HL-60 cells were induced by Bc-4 to differentiate into more mature cells morphologically. The in vivo growth of HL-60 cells in mouse subrenal capsules (SRC) and in diffusion chambers inoculated into mice was inhibited by Bc-4 at a dose of 50 mg/kg. The morphology of HL-60 cells treated by Bc-4 in diffusion chambers exhibited characteristics of mature granulocytic cells.