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The carcinogenic activity of the two polycyclic aromatic hydrocarbons (PAHs), picene (benzo[a]chrysene) and dibenz[a,h]anthracene (DBA), was determined in NMRI mice by five different experimental protocols in order to find out if picene is a carcinogen as predicted by recent quantum mechanical calculations in contrast to earlier observations which could not confirm any carcinogenic activity of picene. Single s.c. treatment of adult mice with picene or DBA (308 nmol/animal, each) led to the formation of fibrosarcomas in 63.3% of treated animals regardless of the PAH used. Chronic epicutaneous application of both PAHs (total dose 1.36 mumol) to the back of mice resulted in the development of papillomas with a tumor rate of 22% in the case of picene and of 32% in the case of DBA. When newborn mice were s.c. treated once on day 2 of their life with each of the two PAHs (400 nmol/animal), 27.8% of treated animals developed lung adenomas after 40 weeks in the case of picene compared to 92.1% in the case of DBA. Histopathological examination of the tumors in the three experimental models revealed no difference in the type of tumor between picene and DBA. Epicutaneous application of both PAHs (600 nmol/animal) followed by chronic treatment with 12-O-tetradecanoyl-phorbol-13-acetate for 24 weeks led to the formation of papillomas in 93% of animals treated with DBA while picene showed no tumorigenic activity at all. Initiation of tumorigenesis in the two-stage tumor model with 7,12-dimethylbenz[a]anthracene (1 mumol/animal) and chronic treatment with picene (total dose 4.8 mumol) for 24 weeks was equally ineffective in producing tumors in NMRI mice. This rare biological property of picene, which is a complete carcinogen, yet at most a very weak tumor initiator, is explained in terms of its inefficient biotransformation to mutagenic and carcinogenic metabolites as compared to the strong tumor initiator DBA.