Milk thistle dietary supplements that contain silymarin are widely marketed and used in the U.S.A and other countries for liver enhancement and recovery. To assess different brands of commercially sold milk thistle, 45 products were collected from local stores and analyzed for their silymarin content and antioxidant activities. High performance thin layer chromatography and high performance liquid chromatography coupled with diode array detector and electrospray mass spectrometry were used for establishing a finger printing and for the determination of all of seven major constituents of silymarin in each product. Antioxidant activity was measured as radical scavenging activity using DPPH and by estimating their antioxidant capacity as trolox equivalent. Samples were found to vary widely in their silymarin content, with some samples having none or very low concentrations while silymarin represented higher than 80% of other samples. Antioxidant activity correlated with the overall level of silymarin in each sample, with samples containing the highest relative concentration of taxifolin exhibiting the most effective antioxidant activity.