5-Methoxypsoralen (5 MOP), the melanogenic component present in several suntan preparations was synthesized and tested by topical applications in inbred XVII nc/Z albino mice combined with 365 nm irradiation with the aim of establishing the relative carcinogenic activity of this compound, as compared to 8-methoxypsoralen (8 MOP) and psoralen. 85% of the animals developed tumors and 25% had multiple tumors. Additional treatment with 12-O-tetradecanoylphorbol-13-acetate raised the tumor incidence to 100%. Tumors caused by 5 MOP show much longer latent periods than those induced by psoralen and 8 MOP. Most of the tumors were rapidly growing squamous cell carcinomas giving metastases in 20% of the animals. The possible long-term effects that might follow the use of 5 MOP in humans are discussed.