The three-dimensional structures of two carcinogens, 7-chloromethyl-12-methylbenz[a]anthracene and 7-chloromethylbenz[ai1anthracene, have been determined by X-ray crystallographic techniques. Both compounds are carcinogenic and are believed to act by alkylating DNA. However, the first has a nonplanar ring system, whereas the second has a planar ring system. The nonplanarity of 7-chloromethyl-12...