Two epimeric carotenoids, named dinochromes A (2) and B (3), were isolated from the fresh water red tide Peridinium bipes, as anti-carcinogenic compounds. The stereostructure of dinochrome A and B were characterized to be (3S,5R,6R,3'S,5'R,8'R)- and (3S,5R,6R,3'S,5'R,8'S)-5',8'-epoxy-6,7-didehydro-5,6,5',8'-tetrahydro-beta,beta-carotene-3,5,3'-triol 3-O-acetate, respectively by (1)H- and (13)C-NMR, and circular dichroism (CD) data. Dinochromes A (2) and B (3) inhibit 12-O-tetradecanoyl phorbol 13-acetate (TPA)-stimulated (32)P-incorporation into the phosholipids of HeLa cells. Furthermore, dinochrome A was found to inhibit the proliferation of human malignant tumor cells, such as GOTO, OST and HeLa cells.