Early carcinogenicity tests found no evidence of activity for picene but found considerable initiating and carcinogenic activity for dibenz[a,h]anthracene (DBA). More recent investigation suggested that both pentacyclics were complete carcinogens when administered as single sc injections in NAMI mice, despite findings that picene acted as neither an initiating nor promoting agent. To investigate this contradiction, the complete carcinogenicities of both isomers were compared by sc injection in female Sprague-Dawley rats. The results demonstrate that 1 mumol of DBA, administered three times weekly for 20 doses, induces sarcomas in all test animals by 33 weeks (100%). Similar treatment with picene did not induce sarcoma in any test animals by 37 weeks (0%). The present results agree with the earlier studies. It follows from these results that the predictions of the unified theory for the appearance of carcinogenic properties following administration of picene and dibenz[a,h]anthracene to test animals have been confirmed. (C) 2002 Elsevier Science.