Some chalcones are known to be phototransformed in a solution from trans into cis isomers. 3-Hydroxy-3'-methylchalcone (3'Me-3-C) has been found to be isomerized from trans into cis by irradiation of daylight in the methanolic solution. The presence of a hydroxyl in the 2'- or 4-position in the trans-chalcone structure prevents phototransformation into cis isomers. The feasibility of phototransformation of chalcones was discussed using UV-spectral analysis. The phototransformed cis-3'Me-3-C showed more potent antitumorigenic activity than the original trans form. The generally recognized parallelism between antitumorigenic and antiinflammatory activities was not observed in trans and cis 3'-Me- and 4'-Me-3C, which are antitumorigenic but inactive in 12-O-tetradecanoylphorbol 13-acetate (TPA)- and arachidonic acid (AA)-induced mouse ear edema. However, inhibitory activity against ornithine decarboxylase (ODC) was commonly observed in both naturally occurring and synthetic antitumorigenic chalcones.