An extract of Spirulina-Dunaliella algae was shown to prevent tumor development in hamster buccal pouch when a 0.1% solution of 7,12-dimethylbenz[a]anthracene (DMBA) in mineral oil was applied topically three times weekly for 28 weeks. The algae extract was delivered by mouth in continued dosages of 140 micrograms in 0.4 ml mineral oil three times per week. After 28 weeks, the animals given vehicle and untreated controls all presented gross tumors of the right buccal pouch. Animals fed canthaxanthin presented a notably and statistically significant reduction in tumor number and size compared with controls. Animals fed beta-carotene demonstrated a smaller but statistically significant reduction in tumor number and size. The algae animals presented a complete absence of gross tumors. However, microscopic sections of the buccal pouch in the algae group showed localized areas of dysplasia and early carcinoma-in-situ undergoing destruction.