Isobolographic analysis of the antiproliferative actions on LNCaP cells in vitro of a mixture of five TCM botanicals and Saw palmetto (Adams et al, 2006) demonstrated that Rabdosia rubescens (RR; Chinese: dongling cao) was the most potent botanical. The major active phytochemical in RR is oridonin (a kaurene diterpene). We compared the tumor inhibitory effects on LAPC‐4 xenografts in SCID mice in the intact and castrate conditions of RR extract (RRE) compared to pure oridonin at equivalent concentrations of oridonin. In intact mice, RRE was more potent than purified oridonin at the equivalent concentration found in RRE and was biologically equivalent to the 5X concentrated oridonin. In the castrate studies, the effects of RRE in reducing tumor growth were greater than the 5X concentration of oridonin. The maximum concentrations in the blood were reached two hours after oral gavage for both RRE and oridonin at equivalent doses were the same. These results clearly demonstrate that the greater potency of RRE by comparsion to oridonin is not due to increased bioavailabilty but presumably due to constituents other than oridonin in RRE. Studies to explore differences in mechanism of action of oridonin and RRE are underway.