Pictures of Ames Tests for (A) Spontaneous Revertants and exposure to (B) Furylfuramide, (C) Alfatoxin B1, and (D) 2-Aminofluorene. The mutagenic compounds in B, C, and D were applied to the 6 mm filter disk in the center of each plate. Each Petri plate contains cells of the tester strain in a thin overlay of top agar. (The strain used here is TA98, derived by adding a resistance transfer factor to a Salmonella tester strain, mutant hisD3052, that scores frameshift mutations.) Plates C and D contain, additionally, a liver microsomal activation system isolated from rats. The spontaneous or compound-induced revertants, each of which reflects a mutational event, appear in a ring as spots around the paper disk. From Bruce N. Ames, Joyce McCann, and Edith Yamasaki, "Methods for Detecting Carcinogens and Mutagens with the Salmonella/Mammalian-Microsomal Mutagenicity Test," Mutation Research 31 (1975): 358.