The effect of algae extract on tumor regression was studied. Phycotene (extract of Spirulina and Dunaliella algae) 250 micrograms in 0.1 ml MEM (minimum essential medium) was injected locally into DMBA (7, 12 dimethylbenz(a)anthracene)-induced squamous cell carcinomas of hamster buccal pouch in 20 animals. DMBA-induced carcinomas in 20 hamsters were injected locally with beta carotene 250 micrograms in 0.1 ml MEM; DMBA-induced carcinomas in 20 animals were injected locally with canthaxanthin, 250 micrograms in 0.1 ml MEM, and DMBA-induced carcinomas in 20 animals were injected locally with 13-cis-retinoic acid, 250 micrograms in 0.1 ml MEM. Twenty animals with DMBA-induced carcinomas were sham-injected controls using 0.1 ml MEM. The various agents were injected into the tumor bearing right buccal pouches twice-weekly for four weeks. Total tumor regression was found in 30% of phycotene animals, 20% of beta carotene animals and 15% of canthaxanthin animals after four weeks. Partial tumor regression was found in the remaining 70% of phycotene animals, 80% of beta carotene animals and 85% of canthaxanthin animals. None of the 13-cis-retinoic acid animals had total tumor regression, but 70% showed partial regression. No tumor regression was found in the DMBA control group and the sham-injected group.