Four major triterpene acids including beta-boswellic acid (1), 3-O-acetyl-beta-boswellic acid (2), 11-keto-beta-boswellic acid (3), and 3-O-acetyl-11-keto-beta-boswellic acid (4) were isolated from the gum resin of Boswellia serrata and examined for their in vitro antitumor activity. They inhibited the synthesis of DNA, RNA and protein in human leukemia HL-60 cells in a dose dependent manner with IC50 values ranging from 0.6 to 7.1 microM. Among them, compound 4 induced the most pronounced inhibitory effects on DNA, RNA and protein synthesis with IC50 values of 0.6, 0.5, and 4.1 microM, respectively. The effect of 4 on DNA synthesis was found to be irreversible. Compound 4 significantly inhibited the cellular growth of HL-60 cells, but did not affect cell viability.