Apoptosis is a new therapeutic target of cancer research. Shikonin isolated from Lithospermum erythrorhizon, a traditional oriental medicinal herb, was observed to induce apoptosis in HL60 human premyelocytic leukemia cell line. Shikonin induced DNA fragmentation into the multiples of 180 bp and increased the percentage of hypodiploid cells in flow cytometry after propidium iodide staining. The increase of apoptotic cells was preceded by the activation of caspase-3, which was reported to play a central role in apoptotic process. The DNA fragmentation induced by shikonin was completely inhibited by the pretreatment of z-VAD-fmk, a specific inhibitor of caspase, clearly showing that the mode of cell death is apoptotic.