The herbal mixture PC-SPES, used for advanced prostate cancer, has proven thrombogenic and highly oestrogenic in clinical trials. However, attempts to identify the active compounds in PC-SPES have yielded incongruous results. Moreover, warfarin was identified in the serum of a patient taking PC-SPES who experienced a bleeding disorder. To determine the active components in PC-SPES potentially responsible for these effects, researchers in the USA analysed PC-SPES lots manufactured from l996 to mid-2001. Antineoplastic activity of PC-SPES and its individual component extracts was determined by colony-forming assays with several prostate cancer cell lines, and oestrogenicity was determined by analysing expression of an oestrogen-responsive reporter gene in breast cancer cells. High-pressure liquid chromatography was used to isolate, identify, and quantify components of PC-SPES. Components were also identified by proton nuclear magnetic resonance, gas chromatography–mass spectrometry, and mass spectra analysis. PC-SPES lots manufactured from 1996 to mid-1999 contained the synthetic compounds indomethacin (range = 1.07–13.19 mg/g) and diethylstilbestrol (range = 107.28–159.27 μg/g) and were two to six times more antineoplastic and up to 50 times more oestrogenic than lots manufactured after the spring of 1999. In lots manufactured after mid-1999, gradual declines in the concentrations of indomethacin (from 1.56 to 0.70 mg/g), diethylstilbestrol (from 46.36 to 0.00 μg/g), and total phytosterols (from 0.586 to 0.085 mg/g) were observed. Warfarin was identified for the first time in lots manufacured after July 1998 (range = 341–560 μg/g). In the August 2001 lot, increases were found in concentrations of the natural products licochalcone A (from 27.6 to 289.2 μg/g) and baicalin (from 12.5 to 38.8 mg/g). The authors concluded that phytochemical composition of PC-SPES varied by lot, and chemical analyses detected various amounts of the synthetic drugs diethylstilbestrol, indomethacin and warfarin, and several natural products. To qualify for clinical pharmacological exploration, nutritional supplements including herbal mixtures should meet standards of quality control under the Good Manufacturing Practice system, and the manufacturers of such supplements should provide reliable analytical quality assurance.