Partially hydrogenated derivatives of several polynuclear hydrocarbons were tested for tumorigenic activity by both skin painting and subcutaneous injection in female Swiss mice. Contrary to previous reports, dibenz[a,c]anthracene was weakly carcinogenic, whereas its 10,11,12,13-tetrahydro derivative was inactive. Dibenz[a,j]anthracene was weakly carcinogenic by skin painting, as was 1,2,3,4-tetrahydro-dibenz[a,j]anthracene, whereas 5,6-dihydrodibenz[a,j]anthracene was much more potent than its unsaturated parent; 1,2,3,4,8,9-hexahydro-dibenz[a,j]anthracene was inactive. This pattern of carcinogenic activity contrasted sharply with that of the analogous series of dibenz[a,h]anthracene derivatives. Two benz[a]anthracene derivatives, 8,9,10,11-tetrahydro-7,12-dimethylbenz[a]anthracene and 11,12-dihydro-3-methylcholanthrene, were both carcinogenic by skin painting, though the latter compound does not have a “K-region.” The results of subcutaneous injection of single doses of dibenz[a,h]anthracene and dibenz[a,j]anthracene and of their corresponding dihydro, tetrahydro, and hexahydro derivatives showed significant tumorigenic activity only by dibenz[a,h]anthracene and 1,2,3,4-tetrahydrodibenz[a,h]anthracene, in contrast to skin painting results.