Two new phenanthroquinolizidine alkaloids, boehmeriasins A and B, were isolated from the aqueous ethanolic extract of Boehmeria siamensis Craib (Urticaceae) by bioassay-guided fractionation. Their structures were elucidated on the basis of spectral evidence. Boehmeriasin A possesses cytotoxic activity against 12 cell lines from 6 panels of cancer including lung can-cer, colon cancer, breast cancer, prostate cancer, kidney cancer and leukemia with GI(50) between 0.2 and 100 ng/mL, whereas boehmeriasin B showed lower activity
