Shikonin, isolated from the plant Lithospermum erythrorhizon Sieb. ET Zucc, inhibited tumor cell growth and induced cell death in various tumor cells, with 50% growth inhibition of human cervical cancer cells, HeLa, at 18.9 +/- 1.1 mumol L-1. Treated with 40 mumol L-1 shikonin, HeLa cells underwent marked apoptotic morphological changes such as a round shape, membrane blebbing and apoptotic bodies derived from the fragmented nuclei. Another hallmark of apoptosis, DNA fragmentation, was observed by gel electrophoresis. Shikonin (10 mumol L-1) significantly blocked the transition from G1 to S phase in the HeLa cell cycle. Pan-caspase inhibitor (Z-VAD-FMK), caspase-3 inhibitor (Z-DEVD-FMK) or caspase-8 inhibitor (Z-IETD-FMK) effectively inhibited shikonin-induced cell death, while caspase-1 inhibitor (Ac-YVAD-CMK) and caspase-9 inhibitor (Z-LEHD-FMK) failed to affect cell death. Caspase-3 activity significantly increased within 12 h after shikonin treatment. Reduced expression of inhibitor of caspase-activated deoxyribonuclease (ICAD) after exposure to shikonin for 12 h suggests the resultant activation of caspase-activated deoxyribonuclease (CAD), leading to apoptosis.