Abstract: Hydroxycamptothecin (HCPT) is an antitumor alkaloid isolated from Camptotheca acuminata Decaisne, a tree native to China. The present investigation showed that HCPT possesses strong cytotoxicity on KB cells using colony formation method. The ED₅₀ was found to be 0.2 μg/ml after 1 h exposure to the drug. HCPT exhibited a significant damaging effect on DNA of KB cells. Its cytotoxicity was shown to be associated with its direct action on DNA strand. It could induce formation of cellular chromatid breaks and increase the frequency of SCE. The effect of HCPT on SCE was found to parallel to the inhibition of colony formation. It was demonstrated that HCPT could induce protein-associated DNA single-strand breaks by means of alkaline elution method. The Km values of DNA elution kinetics of KB cells were found to be 0.04, 0.10, 0.12 and 0.14 h^-1 in the control group and in the treated group with HCPT 10, 50 and 100μg/ml respectively.