Triptolide (Tri) is a diterpenoid triepoxide isolated from Tripterygium wilfordii Hook F. The effects of Tri on the colony formation of breast cancer cell lines MCF-7 and BT-20, stomach cancer cell lines MKN-45, MKN-7, and KATO-III, and promyelocytic leukemia cell line HL-60 were reported. Using Hamburger-Salmon's double layer agar technique with certain modifications, cancer cells were cultured in 0.3% agar in a highly humidified atmosphere of 5% CO2 at 37 degrees C for 14-21 d. Colonies were counted on d 14 (occasionally d 21) with the colony analyzer system CA-7A. Of the 5 solid tumor cell lines tested, 4 showed diminished colony formation in soft agar by greater than 70% of control value in Tri 10(-8) mol.L-1 (continuous exposure). The magnitudes of the inhibitory effect of Tri on most breast and stomach cancer cell lines were similar to that on the leukemia cell line HL-60. IC50 were 0.504-1.22 micrograms.L-1. The clinically achievable peak plasma concentration (PPC) of Tri was estimated as 0.15 mg.L-1, being 72-126 times higher than the IC70 of the cancer cell lines except KATO-III. The results suggest that Tri might have a potential therapeutic effect on some types of solid tumors, e.g., breast and stomach cancers.