Six citrus flavonoids were tested for antineoplastic activity. The hamster cheek pouch model was utilized, and the solutions of the flavonoids (2.0-2.5%) and the solution of the carcinogen, 7,12-dimethylbenz[a]anthracene (0.5%), were applied topically to the pouches. The pouches of the positive controls were treated with the solvent used to dissolve the flavonoids and the solution of the carcinogen. The data show that 4 flavonoids (hesperetin, neohesperidin, tangeretin, and nobiletin) were inactive. The results with naringin and naringenin show that both of these flavonoids significantly lowered tumor number [5.00 (control group), 2.53 (naringin group), and 3.25 (naringenin group)]. Naringin also significantly reduced tumor burden [269 mm(3)(control group) and 77.1 mm(3)(naringin group)]. The data suggest that naringin and naringenin, 2 flavonoids found in high concentrations in grapefruit, may be able to inhibit the development of cancer.