For 80 weeks, C3H male mice were given repeated topical applications of 6 polycyclic aromatic hydrocarbons-chrysene (C), fluoranthene (F), pyrene, triphenylene (T), perylene (P), and benzo[b]triphenylene (BT)-in either decahydronaphthalene (Decalin), a noncocarcinogenic vehicle, or 50: 50 Decalin:n-dodecane, a cocarcinogenic mixture. When applied in Decalin, only BT produced malignant tumors. When combined with 50: 50 Decalin:n-dodecane, C, T, and pyrene, but not BT, produced malignant tumors. C,in particular, was outstanding in its production of tumors with the help of dodecane. C and T were classified as incomplete carcinogens, lacking any cocarcinogenic activity. F and P were inactive under these conditions. The 5-ringed BT behaved entirely differently from C and T; i.e., its activity was decreased when n-dodecane wasintroduced. Tentative estimates were made of the relative contribution of cocarcinogenic versus chrysene-Iike activity to the overall activity in carcinogenesis of each of the 6 aromatic hydrocarbons and of several others frequently tested.