Oridonin, a diterpenoid isolated from Rabdosia rubescens (Hemsl.) Hara, inhibited the growth of human tumor cell lines SW620 (colon), MCF-7 (breast) and K562 (bone marrow), and induced significant levels of apoptosis in SW620. Morphological changes indicative of cell apoptosis were observed after the cells were exposed to oridonin for 24 h. Growth inhibition was associated with G1 phase arrest, and with time- and dose-dependent increases in caspase-3 activity. We therefore conclude that oridonin inhibits the proliferation of SW620 cells by induction of apoptosis via the activation of caspase-3. Our data suggest that oridonin may have significant potential as an anti-colorectal adenocarcinoma agent.